A comic book character from Marvel Comics. His first appearance was Iron Man #55.
He is one of the Eternals of Titan, the moon of Saturn. As an Eternal, he had all sorts of nifty powers, like super strength, energy blasts, and increased intelligence. When he first appeared in comics, he was in love with Death (the physical entity). As a result, he spent a lot of his time killing people. But Thanos was not one to be contented with mere butchery. He set his sights much higher. A nihilist, he decided that the meaninglessness of reality gave him license to end it.
Thanos first tried to take over/destroy the universe using the Cosmic Cube, but was stopped by Captain Marvel and Drax the Destroyer. Not one to quit after a minor setback, he attempted to do so a second time by exploding every star in the universe at once. A contingent of Earth's heroes stopped him. By the end of the battle, Thanos had killed Adam Warlock, but Adam Warlock's soul turned him into stone, effectively killing him. Later, Death, aware of Thanos' devotion to her, resurrected him in a greatly improved form so he could kill off half the universe. Thanos's new abilities included a massive increase in energy and strength, and extreme durability. He could even survive in a vacuum. Thanos decided not to use his new strength to just go around killing people. Instead, he gathered together the six Infinity gems and effectively became God. Using the Infinity Gauntlet, he blinked half of the sentient beings in the universe out of existence and proved his dominion over the universe, but lost this dominion after weathering several assaults on his power. Brute force did not win the day; really it all came down to one thing: Thanos had a subconscious flaw which made him think he was unworthy of Death's affections, so he could never carry out his plans to appease her (such as destroying the universe). He's over that flaw now, but after being spurned by Death, he's also over the desire to destroy the universe. Funny how these things work out. The Infinity Gauntlet debacle more or less brought an end to Thanos' nihilism, which was replaced by a cold, calculating pragmatism. Now we saw a Thanos who was prepared for every eventuality, and had the resources to know about every important development in the universe.
He was (sort of) a member of The Infinity Watch in that he guarded the reality gem. This was bestowed upon him by Adam Warlock, who realized that Thanos was smart enough not to mess with reality using the gem, but strong enough to keep others from attempting just that. Now, the gems are out of the picture.
Thanos was thought to be dead, killed by Thor in a fight. However, this turned out to be a clone of his. Shortly after his supposed death, several of Thanos' other clones attempted to kill him and destroy reality. They had inherited all of his nihilistic tendencies and none of his pragmatism. After dispatching of his clones (and saving the universe in the process), Thanos spent some time looking for more power.
Of course he was successful. He managed to become God (again, only moreso this time) only to destroy the universe in a fit of rage. This was a bad thing, other than the kiss it earned him from Death. Reality existing or no, Thanos still had his omnipotence, and so decided to bring back the universe. The catch was that in order to re-create it, he had to relinquish his omnipotence. His stint as God had changed him, however, and he set about becoming a force for good in his own ruthless way.