Chōjin (超人, Supermen) is the Japanese word for Supermen. It is also the name of a fictional race from Yudetamago's manga and anime series Kinnikuman and its sequel Kinnikuman Nisei. This article focuses on the latter meaning.
Chojin are people whose abilities far surpass that of normal humans. Most, like Kinnikuman and Terryman, were born with these abilities while some, like Geronimo and the Brocken Clan achieved it through special means. Most of them reside on Earth. In earlier chapters they were known as Superheroes (スーパーヒーロー), but it was changed to Chōjin after the first Chojin Olympics.
In Nisei, the Brutal, Devil, Perfect, and Time Chojin are collectively referred to as Devil Chojin (悪行超人, Akugyo Supermen). There are also anime and movie original chojin such as the Goku'aku Chojin, Kakure Chojin, and the Senshi Chōjin.
The ancestral origins of the chojin breed predate humanity, and most of the iconic artefacts and features of the world are traces of the chojin passage (Ayers Rock being an ancient arena, the Leaning Tower of Pisa built in imitation of a tower overseeing an ancient ring in Campo dei Miracoli).
The very first chojin civilisation was described as noble and just, living under the chojin Gods' rule, until their pride in their physical prowess led them to quarrel and fight.  They adopted a Might is Right mindset with the strongest chojin oppressing and discriminating the weakest ones, building a highly elitist society marred by pride, greed and envy. This caused the Chojin Gods to decide upon wiping out the entire chojin race, returning the mortal plane to a vacant and lifeless lot by bathing the universe in the deadly Capillaria Ray, a godly energy able to melt the flesh of the Chojin sparing every other life form. 
A Merciful God rose among them, pleading to be allowed to save the most righteous Chojin and teach them how to be a new [Perfect Origin] for a new wave of their species: the God allowed the Merciful God to descend in the mortal plane, choose some Chojin and carry them in a space between Heaven and Earth. 
The Merciful God, unable to return to Heaven, became The Man, aka Chōjin Enma, aka Perfect Zero and taught his newly christened Perfect Origin with great hope. Unbeknownst to the Chojin Gods and the Merciful God, some of the Chojin fated to die managed to survive the Capillaria Disaster, hiding under the branches of a mystical tree on Earth, the Unforgiving Yggdrasil. In that moment. the Perfect Origin broke as dissent spread among his elements: while The Man still held in his original belief than only the Perfect Origin could be considered worthy of creating a Chojin Race obedient to the Chojin Gods, the two brothers Goldman and Silverman, the closest allies and disciples of The Man, left his side to reach Earth.
Goldman and Silverman took great interest in the surviving chojin, and wished to teach them, feeling the Merciful God had lost his mercy and had become obsessed into denying them a new lease at life. Goldman left first, then Psychoman asked Silverman to follow his brother and spy on him: however Silverman learnt by Goldman about the untapped possibilities of the chojin left in the mortal plane and decided to stay there.
Goldman then taught those who, in the future, would become the Akuma Chojin while Silverman, believing peace and mercy had to be preserved, started teaching the Justice Chojin and, by taking a family in the mortal plane, became the forefather of the Kinniku Clan, natural-born leaders of the Justice Chojin themselves.
Despite The Man and Psychoman homed the Justice Chojin and the Kinniku Clan to be loyal servants of the Perfect Origin and just a tool to keep Goldman in check, Silverman had different plans for them, hoping they could rediscover the ideal of Mercy and Justice the first generation of Chōjin lost.
Goldman and Silverman eventually fought one last time, under Justiceman's watch: the Quarrel in Heaven ended up with the two brothers decapitating each other, becoming the Gold and Silver Mask kept in custody by the Kinniku Clan, inheritors of Silverman's teaching but oblivious to their connections to the Perfect Origin. A series of lies and fabrications were put in the Chōjin Archives, describing Goldman and Silverman as Chojin Gods on their own, unrelated to Chōjin Enma, and allowing the Devil and Justice Chojin to flourish and rebuild their society.
In the subsequent millennia, chojin and humans started living side by side, with the Justice Chojin acting as heroes and protectors and the Devil Chojin as villains and heels caring only for battle.
By modern times, or rather the eighties, Chojins were a common sight on Earth and the Cosmos, with entire planets populated by Chojin themselves, like Planet Kinniku, and with Earth hosting one or more Chojin for every nation, acting as protectors of humanity.
Silverman's inheritance resurfaced in that era when his latest descendant, Suguru Kinnikuman, prince of the Kinniku Clan, developed the Fire of Inner Strength and the Friendship Power, two faces of the same coin, the ability to reach a greater strength in times of need and for the sake of justice and friendship.
Such a power, and the possibility that Kinnikuman could spread it among his brethren, piqued the interest of the Evil Gods, sending the Fated Princes to strip Kinnikuman of his royal inheritance and the The Man himself, who sent the Perfect Chojin to deal with Kinnikuman and attempt to eradicate the Fire of Inner Strength and the Friendship Power forever.
Finally, Kinnikuman managed to ascend to the throne, defeat the Fated Princes and the Perfect Origin itself.
It took Goldman however, now known as General Devil to finally defeat his former master Strong the Budo, aka The Man, aka Chojin Enma. However, Kinnikuman in a display of his ideals of Mercy begged Akuma Shogun to spare The Man, letting him return to the Chojin Graveyard.
Finally, Chojin Enma remembered his former Mercy, and accepted. However, before returning forever to his role as a God of the Underworld, he warned Kinnikuman that, if her ever strayed from his ideals of Mercy, he would send his disciples, now stationed in the Graveyard, to check on him.
In the future continuity (actually, the 2010s) of Kinnikuman Nisei a lengthy period of peace left the Chojin weak, living as peaceful, untrained human beings on their respective planets: when a faction of Akuma, Perfect and Brutal Chojin rebuilt the dMp to wage war to humanity, the Muscle League, the alliance of Justice Chojin decided to train their newest members, leaving to Mantaro Kinniku, son of Kinnikuman, the role to rediscover the ancestral power of his family and lead the new generation.
There's no uniformity in the physical features of the average chojin. A chojin may look perfectly human, as Terryman and his son, humanoid with some non-human features (like Buffaloman with his Long Horns or the faceless Black Hole) or humanoid with distorted features (as the comically stereotypical Ramenman). He may also look as a common appliance, like Mixer Taitei, Benkiman and Stereo Cassette King, some kind of monster, like Gepparland and Sneagator, a robot (like the cyborg Warsman), or being an utterly non-human creature, like Sunshine.
The Kinniku Clan is composed by fully human looking muscular beings, however they're forced to wear an iconic, Ultraman-like mask on their faces, never meant to be shown to anyone else: if they did, they'd be sentenced to death (or, in the American dub of Kinnikuman Nisei, stripped of their rank, status, forbidden to wrestle or fight again and reduced, by every standard, to un-persons without any right).
Chojin are usually far stronger than human beings,  save for some exceptions, as the comically weak Kani Base, the bumbling Kinkotsuman and the very early Kinnikuman himself. It is said that chojin aren't any different from humans, until they fight, and - at this point - their chojin power takes effect. 
Most of them exhibit stereotypical superhuman abilities: in the earliest part of the series the wide majority of chojin, included Terryman and Kinnikuman themselves, were shown able to grow in size and fly through the air. Those powers were never mentioned or seen again when the series shifted to a story based on wrestling, but the chojin were left some other abilities tied to their physical form, like Buffaloman's ability to manipulate his horns or Sneagator's ability to transform, or Pentagon's ability to fly using his inborn wings and alter time and space themselves.
A chojin's powers are defined by their Chojin Kyodo, a number measuring their inborn power. The Kyodo isn't an absolute measure though: a chojin like Canadianman, for example, is shown as strong in its kyodo, but lacking in technique and abilities, thus inferior to lesser chojin whose kyodo is lesser, but with more technique and finesse. Kinnikuman has shown the to increase his kyodo when in a pinch, and other chojins have developed forms of the same ability.
Chojin are physically stronger and more fit than humans, able to withstand a bigger deal of punishment and heal much faster:  Ramenman shrugged in a few days wounds that a human doctor described as needing months to heal. This ability may be linked to their power and training, as an earlier Kinnikuman witnessing Ramenman's healing quipped that he takes even more time than a human to heal, but as Kinnikuman is a bumbling, happy-go-lucky joker, it could also be read as a joke.
Even a fully human chojin is fundamentally different from a human being: Ramenman broke a MRA machine and other human diagnostic devices just by being examined, explaining the doctors that human western medicine was no good for an eastern chojin like him, and chojin bodies in the series were shown able to withstand surgical and medical procedures impossible for a human being (like Kinnikuman getting a prosthetic bone made by Buffaloman long horns, or Terryman getting a fully functional prosthetic leg made by his farrier).
Despite this, chojin and human may interbreed, and the result can be both a human, both a chojin, but never a crossbreed. Terryman's son, Terry the Kid, is the half Japanese son of the chojin Terryman and the human Natsuko Shono, and he's a full-fledged chojin, while Victory Ramenman's sister is fully human.
Chojin and human beings remain though two separate species, tied to a different afterlife and even subjected to different gods: some chojin however are born as humans. The Brocken Clan, for example, is made by human beings that, after a gruelling training, are granted a mystical item (a skull or a small eagle, according to the continuity) granting them a chojin body, and after his physical death the human Geronimo was granted from the Human God the boon to be able to meet in spirit a Chōjin God and petition him to be reborn as a chojin.
Souls and Afterlife
Chojin possess an immortal soul, and they're subjected to several gods: after their death however their souls are sent to the Chojin Graveyard, overseen by the fallen god Chojin Enma, first of the Perfect Chojin, where they can be revived by gathering enough Chojin Kyodo turning the Mortier du Pileur, an ancestral machinery able to turn magnetic power into Chojin Kyodo. As for the Perfect Origin Arc, Akuma Shogun forbid Chōjin Enma from using it ever again, damaging the Mortier du Pileur beyond repair and prompting Enma to become again a righteous god caring for the fallen souls of the Chojin and granting them eternal rest.
A chojin can - however - still be revived if enough chojin spare with him their kyodo, as chojin can transfuse their power (just like giving blood),  and they can also be revived by the Face Flash, the elusive power that some talented members of the Kinniku Clan may shine on their bodies. Is still uncertain if General Devil's ban on resurrection affects those ways to cheat death too.
Save for Kinnikuman Lady and Kinnikuman Nisei ~All Chojin Dai Shingeki~, the series lack any actively fighting female chojin, the only notable exception being possibly Bibimba. It is worth noting, within the anime, it is stated that Chojin are 'neither male or female', and that is possible that chojin have a different concept of sex and/or gender. In the American dub of the anime, Jacqueline expresses a desire to wrestle, as such it is possible that there could be cultural reasons among chojin why female-identifying chojin are not seen to battle, despite it being explicitly shown in various manga series that it is possible for them to fight. 
Chojin are usually organised among their groups.
The Justice Chojin answer to a central Committee, in which the Kinniku Clan has always a prominent role, deciding about their roles, position and ruling over their actions. However, in the earliest part of the series, when the chojin were monster hunters for hire, they answered to their governments.
The more experienced Chojin are meant to teach the younger ones, and, as told by General Devil and Silverman. The younger chojin are also taught in chojin schools throughout the world, although this seems to have been retconned into just one establishment in Kinnikuman Nisei: the Hercules Factory. When an old Justice Chojin is too old to continue fighting or being able to serve in any facility, he's given an ornate walking cane as a prize for his efforts.
The Devil Chojin answer to a looser organisation, usually flocking around the strongest and more charismatic of them, like Akuma Shogun, Buffaloman when he is a Devil Chojin and not a Justice Chojin or Sunshine.
At some point in time, Horae Avenir was born among their people. He is said to have been a man with a great sense of justice, which led him to become a leader of his people, and he created a clan of Justice-Time Chojin. They left the Chojin Prisoner Planet with Horae as their leader, and headed to a new planet that they went on to name Planet Avenir. 
At first, no crops would grow or thrive on this planet, but Horae inspired his people to cultivate the land and to grow crops that were plentiful and desired. The Time Chojin grew to be a prosperous, peaceful, and happy people; Horae Minutes passed peacefully in his sleep, and eventually - after two-hundred years - Minutes Avenir became king of their people. They create their own form of wrestling for entertainment. 
A chojin's outfit - whether shorts, tights, or a body-suit - is said to be a chojin's life; when Kinnikuman Soldier rips his attire, it thus causes immense shock to Brocken Jr. 
Main Chojin Classifications
Justice Chojin (正義超人, Seigi Chojin) are Chojin who use their powers for the sake of justice, mainly battling Kaiju in earlier chapters. There are said to be 1,000 of them on Earth.
Friendship is absolute for Justice Chojin, and if for some reason a crack begins to appear in their friendship the ancient Iron Sweat (アイアン・スエット) begins to flow from their bodies. It continues to flow and weigh them down until their friendship is restored.
Jikan Chojin (時間超人, Time Supermen) are mysterious Chojin whose true identities are unknown. A mutated breed of Evil Chojin, they have revealed themselves to Meat and the others, but their origin and background is still unknown. When the Majikaku (魔時角, Demonic Time Horn) that sprout from their heads are broken off, they gain the ability to travel a single period of time from within their own lifetimes. At present, only Thunder and Lightning have appeared (although it has been hinted that Chaos may be one as well). They also have the ability to transform parts of their bodies into pieces of a clock (bells, minute hands, etc.), but whether this is an ability exclusive to the Time Chojin is unknown.
Brutal Chojin (残虐超人, Zangyaku Chojin) are Chojin who fight brutally and kill their opponents for their own amusement. They are basically Justice Chojin gone bad, and are even allowed to compete in the Chojin Olympics. Later on, they are divided between those who aid the Justice Chojins (Seigi no Zangyaku Chojin (正義の残虐超人, "Good" Brutal Supermen). This includes Ramenman, Ilioukhine, Warsman, Kinkotsuman, and others. Although very brutal after entering the ring, they are essentially Justice Chojins) and those who oppose them (Aku no Zangyaku Chojin (悪の残虐超人, "Evil" Brutal Supermen), this include the entire True Team Soldier, Shimaou, and others. They mostly appear in the sequel series).
Devil Chojin (悪魔超人, Akuma Chojin) are Chojin who have sold their bodies, minds, and blood to demons, and fight under the control of Great Demon King Satan (大魔王サタン, Daimaō Satan). To them, winning means killing your opponent and losing means "death". Ex: Buffaloman, Springman, Asuraman, etc.
Originally the Brutal Chojin were included with the Devil Chojin, but this was later changed.
Perfect Chojin (完璧超人) (Read as Kanpeki Chojin in the first anime, Kanpeki being the non-ateji reading of the character 完璧) are Chojin who were so strong that nobody in this world could oppose them, so they exiled themselves to the heavens. There they sparred with each other for thousands of years, transcending emotions and gaining power that rivalled the gods. They follow many strict rules, such as "Never use weapons", "Never show your back to an enemy", and "Defeat is absolutely inexcusable". Ex: Neptuneman, Neptune King, Seiuchin, etc.
Classified by Unique Groups
The Five Fated Princes (運命の五王子, Unmei no Go Ouji) are five chojin who are succession candidates for the Planet Kinniku Throne and are being controlled by the Evil Chojin Gods. Because the possess the powers of a god, each one has a Chojin Kyodo of 100,000,000 Power. Ex: Super Phoenix, Zebra, Big Body, Mariposa, Soldier.
Most of the Chojin who compete on the Five Fated Prince's teams are never classified as a specific Chojin. Some exceptions are Team Phoenix's The Omegaman (Perfect Chojin), Team Phoenix's Satan Cross, (Devil Chojin), and the members of the True Team Soldier (Brutal Chojin).
Classified by Physical Features, Jobs, etc
These are chojin that are part robotic in nature, such as Warsman and Meltdown. They usually have characteristics based on electronic items or mechanical items, or are designed after robots or cyborgs, and - despite sometimes being fully non-organic (or partially non-organic) they can still reproduce. 
Warsman describes these chojin as being calm in nature, as well as having a refusal to lose to 'living' chojin, and states that their wild instinct can often impede their judgement in battle. 
Masked Men are, as the name implies, supermen who were masks as part of their costume, or to conceal their identity. Some Chojin, like Kinnikuman and Robin Mask, have been masked since birth or childhood as part of a family tradition. While others, like Neptuneman are masked to empower themselves.
Yōhei Chōjin (傭兵超人, Mercenary Supermen) are, as their name implies, Chojin who work as mercenaries. The only confirmed one is The Hawkman, however Team Zebra member Parthenon, who had only joined with Zebra for money and a cabinet position, could possibly be one as well.