Kryptonians had evolved to absorb and store solar energy and to tolerate high-gravity environments that would immobilize or even kill weaker species.
While on Krypton, which was fifteen times as massive as Earth and orbited a relatively low-heat Red Giant (or in some versions Red Dwarf), their physical abilities were about identical to humans.
Naturally, the Clark Kent/Superman dichotomy—most particularly, the question of which is the "real" person and which is the "mask"—has been explored a great deal, and has changed over time (with Kent going from nervous, geeky klutz to sharp-witted Intrepid Reporter, among other changes).
In the Golden and the Silver Age, Clark Kent was little more than a façade for Superman. Both sides also tend to be a lot more psychologically/emotionally vulnerable than you'd expect.
In DC/Marvel crossover events, his counterpart/opponent is normally The Incredible Hulk as Hulk is one of the few Marvel characters who's a physical match for him.
See also Supergirl, his Distaff Counterpart, and Krypto the Superdog, his Kryptonian dog., a graphic novel written by Steven T.
Along with Batman and Wonder Woman, he's one of the Big Three of The DCU.
He has also been a member of the Justice League of America on and off (mostly on) since its founding.
In other takes, Superman has been a bachelor or dated/married different characters (in the New 52, he was dating Wonder Woman).
Originally created by Jerry Siegel (writer) and Joe Shuster (artist), two sons of Jewish immigrants who, after several tries, finally got him published in This wasn't their first attempt at the character they had in mind.
It was in the latter adaptation that Superman received his most iconic superpower, that of flight.