Superman, given the serial nature of comic publishing and the length of the character's existence, has evolved as a character as his adventures have increased. The details of Superman's origin, relationships and abilities changed significantly during the course of the character's publication, from what is considered the Golden Age of comic books through the Modern Age. The powers and villains were developed through the 1940s, with Superman developing the ability to fly, and costumed villains introduced from 1941. The character was shown as learning of the existence of Krypton in 1949. The concept itself had originally been established to the reader in 1939, in the Superman comic strip.
The 1960s saw the introduction of a second Superman, Kal-L. DC had established a multiverse within the fictional universe its characters shared. This allowed characters published in the 1940s to exist alongside updated counterparts published in the 1960s. This was explained to the reader through the notion that the two groups of characters inhabited parallel Earths. The second Superman was introduced to explain to the reader Superman's membership of both the 1940s superhero team the Justice Society of America and the 1960s superhero team the Justice League of America.
Superman dies in Lois Lane's arms: Superman vol. 2, #75 (Jan. 1993). Art by Dan Jurgens& Brett Breeding.
The 1980s saw radical revisions of the character. DC Comics decided to remove the multiverse in a bid to simplify its comics line. This led to the rewriting of the back story of the characters DC published, Superman included. John Byrne rewrote Superman, removing many established conventions and characters from continuity, including Superboy and Supergirl. Byrne also re-established Superman's adoptive parents, The Kents, as characters. In the previous continuity the characters had been written as having died early in Superman's life (about the time of Clark Kent's graduation from high school).
The 1990s saw Superman killed by the villain Doomsday, although the character was soon resurrected. Superman also marries Lois Lane in 1996. His origin is again revisited in 2004. In 2006 Superman is stripped of his powers, although these are restored within a fictional year.
Personality
In the original Siegel and Shuster stories, Superman's personality is rough and aggressive. The character was seen stepping in to stop wife beaters, profiteers, a lynch mob and gangsters, with rather rough edges and a looser moral code than audiences may be used to today. Later writers have softened the character, and instilled a sense of idealism and moral code of conduct. Although not as cold-blooded as the early Batman, the Superman featured in the comics of the 1930s is unconcerned about the harm his strength may cause, tossing villainous characters in such a manner that fatalities would presumably occur, although these were seldom shown explicitly on the page. This came to an end late in 1940, when new editor Whitney Ellsworth instituted a code of conduct for his characters to follow, banning Superman from ever killing.
Today, Superman adheres to a strict moral code, often attributed to the Midwestern values with which he was raised. His commitment to operating within the law has been an example to many other heroes but has stirred resentment among others, who refer to him as the "big blue boy scout."
Powers
As originally conceived and presented in his early stories, Superman's powers were relatively limited, consisting of superhuman strength that allowed him to lift a car over his head, run at amazing speeds and leap one-eighth of a mile, as well as incredibly tough skin that could be pierced by nothing less than an exploding artillery shell. Siegel and Shuster compared his strength and leaping abilities to an ant and a grasshopper. When making the cartoons, the Fleischer Brothers found it difficult to keep animating him leaping and requested to DC to change his ability to flying. Writers gradually increased his powers to larger extents during the Silver Age, in which Superman could fly to other worlds and galaxies and even across universes with relative ease. He would often fly across the solar system to stop meteors from hitting the Earth, or sometimes just to clear his head. Writers found it increasingly difficult to write Superman stories in which the character was believably challenged, so DC Comics made a series of attempts to rein the character in. The most significant attempt, John Byrne's 1986 rewrite, established several hard limits on his abilities: He barely survives a nuclear blast, and his space flights are limited by how long he can hold his breath. Superman's power levels have again increased since then, with Superman currently possessing enough strength to hurl mountains, withstand nuclear blasts with ease, fly into the sun unharmed, and survive in the vacuum of outer space without oxygen.
The source of Superman's powers has changed subtly over the course of his history. It was originally stated that Superman's abilities derived from his Kryptonian heritage, which made him eons more evolved than humans. This was soon amended, with the source for the powers now based upon the establishment of Krypton's gravity as having been stronger than that of the Earth. This situation mirrors that of Edgar Rice Burroughs' John Carter. As Superman's powers increased, the implication that all Kryptonians had possessed the same abilities became problematic for writers, making it doubtful that a race of such beings could have been wiped out by something as trifling as an exploding planet. In part to counter this, the Superman writers established that Kryptonians, whose native star Rao had been red, only possessed superpowers under the light of a yellow sun. More recent stories have attempted to find a balance between the two explanations.
source: wiki