Doomsday is the name of a fictional supervillain in the Superman comic book series. He is generally considered to be the DC comics version of the Incredible Hulk.
History
Doomsday was created in 1992 by writer/artist Dan Jurgens for a story arc in the Superman series of comic books that was titled The Death of Superman. In this series, Doomsday was an ultra-powerful destructive villain who engaged in a seemingly endless, mindless rampage across the face of the United States. He had been buried underground for an unknown period of time at the beginning of the story arc, until he tunneled his way out and into the light of day. He immediately began attacking anything and everything within range, with the mindless ferocity of a monster, never speaking, and apparently enjoying the wanton destruction and mayhem he was causing.
In his first encounter with the Justice League, Doomsday defeated the entire team of superheroes in a matter of minutes, laying waste to the entire surrounding area, and finally attracting the attention of Superman. (Because he never spoke, he never called himself "Doomsday." The name "Doomsday" was coined by Booster Gold, one of the members of the Justice League, who said that the monster's rampage resembled "the arrival of Doomsday," meaning the end of the world.) Superman quickly found that Doomsday's awesome power was easily a match for his own, and he realized that if Doomsday reached the city of Metropolis, the resulting battle could conceivably destroy the city and kill millions of innocent people.
In the space of only a few issues of the Superman comic book series, Superman and Doomsday engaged in a titanic, no-holds-barred battle that made Superman realize that Doomsday would continue to attack relentlessly, unending, with no defeat or surrender in sight. It culminated in Superman #75, where in a heroic act of self-sacrifice, Superman fought Doomsday to the bitter end, until the two combatants struck a simultaneous, fatal blow that left both combatants lifeless.
In the aftermath of Superman's apparent death, no less than four super-beings appeared in his wake, all of them declaring themselves to be the "real" Superman. One of these four, a half-man/half-machine cyborg (who would later become a dangerous villain called simply "The Cyborg") took the apparently lifeless body of Doomsday and flung it into deep space, on a trajectory that would supposedly ensure that it would never land on any planet.
However, Doomsday's body was found by a deep-space scavenger ship that happened to be en route to Apokolips, the home of the evil Darkseid. During this trip it was learned that Doomsday was not really dead. He revived and, after slaughtering the crew of the salvage ship, found himself on the harsh world of Apokolips. This was the setup for a final showdown between Doomsday and Superman, as Superman had also returned from the dead by this time.
It was found that Doomsday was artificially created by Bertron, a mad scientist working on Krypton, Superman's home world, though neither he nor his creator were Kryptonian (although Bertron had used a cadre of Kryptonian scientists who assisted him in his lab). Doomsday's hatred toward Superman was due to the fact that whenever he looked at a Kryptonian, he saw his creator who had subjected him to harsh environments over and over again in order to create the perfect life form; Bertron would typically create a lifeform, then send it out into the harsh Kryptonian environment where it would be quickly killed. Each time this occurred, Bertron ordered the lifeform's remains to be harvested and used again, to create a better, stronger version of the last. Thus, the being which would eventually become Doomsday was forced to endure the agony of death, thousands upon thousands of times; its memory of these countless deaths drove it to hate all life, and especially all Kryptonians.
Inevitably, Doomsday continued to return, menacing Superman and the other heroes of Earth time and again. On one occasion Doomsday was recruited by Lex Luthor's Suicide Squad to battle Imperiex, a threat that was judged to be even greater than Doomsday himself. Later Doomsday returned to slug it out with Superman in another one on one slugfest and got overpowered and defeated proving that the Man of Steel is far superior to the beast.
Doomsday in Other Media
In the animated television series, Justice League, the Justice Lords (a tyranical version of the Justice League from a parallel universe) fought a weaker, more intelligent version of Doomsday. The situation was resolved by that version of Superman firing his heat vision into the villain's head, lobotomizing him. Later, it was revealed that this canon's Doomsday was created as an imperfect clone of Superman and programmed to hate the Man of Steel. His brain eventually regenerated, and he resumed his quest to kill Superman. He was finally banished to the Phantom Zone as a last resort.