He is what he is, but he isn't what he used to be. How many versions of Kal-El, aka Clark (Superman) Kent have there been since he first appeared in 1938's Action Comics #1? I'm looking for the subset of canon Supermen and the differences between them, I'm not looking for a list of actors. For instance, after 1992's "The Death of Superman", he came back in several different incarnations.
This list does not include every alternate version of Superman, nor does it deal with his first animated appearances in the 1940's Fleisher Superman Adventures, The New Adventures of Superman series from the 1960s, his appearances in the Super Friends in the 1970s and 1980s, his award winning Superman: The Animated Series in the 1990s or the Justice League and Justice League Unlimited appearances. Nor does it include the All-Star Superman who is an amalgam of all of the printed versions of Superman bound into one.
Earth-2 Pre-Crisis, Golden Age, Superman, Kal-L - deceased, removed from continuity during Crisis on Infinite Earths; Killed in the second Infinite Crisis Saga; Reanimated for Blackest Day Saga. What a crappy way to treat the greatest hero ever!
Earth-1 Pre-Crisis, Silver Age, Superman - removed from continuity during the recent reboot of the DCnU. Longest lasting version of the character, married to Lois Lane. Most well-known version of the character. Also the version used for the movie versions of the character.
Earth-1 Pre-Crisis, Silver Age, Superboy - lasted for 40 years, was an integral part of the Legion of Superheroes, both as an inspiration for the formation of the group and later as a long-time member. He was eventually retconned out of the DCU during the Crisis on Infinite Earth event. Replaced with hybrid Superboy, Conner Kent, a clone of Superman and Lex Luthor and made by Project Cadmus.
"Mullet Superman" - not a new character but brought back from the dead, more powerful than ever. After Superman returned to life ending the Reign of the Supermen storyline, he sported a distinctive and often disliked mullet haircut. Superman was also significantly stronger after his "death."
Superman Red and Superman Blue: Superman becomes an energy being and is split into two distinct beings. An old story from the Curt Swan era where Superman is split into two beings is redone in a modern theme giving Kal-El distinctively new powers based in electromagnetic manipulation. Commonly called the Electric Superman, most readers disliked the transformation intensely and could not wait until Classic Superman was restored.
Superman 1,000,000, Kal Kent, a descendant of Superman from the 853 century and leader of the Justice Legion Alpha. This version of Superman has powers on par with the Silver Age Superman at his peak of power. As to whether this version of the character and the Superman-Prime version of the character retain canon, they were both in a canon series of stories which were ultimately erased at the end of the Infinite Crisis and the establishment of the New Earth/New 52 Universe.
Superman-Prime - Kal-El; having retired to living in the center of the Sun, this version of Superman has moved away from a human existence and lends his powers to his descendants. He is a god-like being of immense power.
Superboy-Prime; Earth-Prime, Infinite-Crisis - A version of the character from an Earth without superheroes. Recently went mad and was believed destroyed during the Countdown to Final Crisis Saga. After some introspective wrangling back on Earth Prime, he eventually is driven mad and imprisoned in the Source Wall. Good riddance. One of the saddest character conversions as well as one of the least well written stories, ever. Superboy-Prime started as a completely sympathetic character turned into one of the worst parody villains in DC history.
The Kingdom Superman, a famous alternative Superman and future history that was later merged into the primary storyline during the Hypertime Years. This Superman retires after the murder of Lois Lane and while he does, more nihilistic heroes take up the reigns. His return to the struggle causes epic upheavals on that world. Award-winning artwork by Alex Ross and story by Mark Waid.
DCnU - New 52 - Earth-0, Superman: Also known as Armored Superman (Superman) or Blue-Jean Superman (Action Comics). This is the latest version of Superman since DC has started their latest reboot of the DC new Universe called the DCnU or the New 52, denoting 52 separate universes that survived their previous reboot. Much of what we know about Superman has been retained with the obvious exceptions of his involvement with Lois Lane (she has, at least temporarily) been supplanted with his interest in Wonder Woman and his costume losing its briefs and becoming a suit of nano-technological armor.
Val-Zod - New 52 - Earth-2: Replaces Earth-2's Kal-El as Superman, Val-Zod was educated as a pacifist and while Kryptonian, with all powers and abilities expected, he was reluctant to use his powers for a time, until Kal-El was killed.
Kalel - New 52 - Earth-23 - Multiversity Series: Born on Krypton, in the science-capital Jandra-La on Vathlo Island in the green Dandahu Ocean, Jorel and Lara send their only son to Earth to escape the cataclysm. Raised by a poor couple Calvin Ellis would one day rise to the office of the presidency and become the greatest superhero on Earth-23, Superman. This version of Superman is strongly based on both real-life President Barack Obama and famed boxer Muhammad Ali (as confirmed by writer Grant Morrison).