On May 4th, 2012, millions of watchers stayed after the credits of The Avengers to see catch a glimpse of a menacing purple-faced dude with an evil smirk. This article is going to answer the question that about ninety-percent of viewers asked: "Who is that?"
That, my good people, is Thanos.
Thanos was born on Titan, one of Saturn's moons. He lived in a colony of Eternals, where his deformed and monstrous stature made him more or less an outcast among his own people. He became obsessed with the concept of death, and as he grew older, he used cybernetic implants to make himself more powerful.
After being exiled from his home planet, Thanos traveled the universe, steadily becoming more and more powerful. During his travels, he met Death herself, and quickly fell in love with her. Seeking to please her, he built an army an marched on Titan itself and declared himself its ruler.
He then located and equipped the Cosmic Cube. Naming himself a god, he successfully fought off Captain Marvel and the Avengers. Captain Marvell managed to trick Thanos into believing that the Cosmic Cube had lost its energy, and Thanos discarded it. Captain Marvel quickly grabbed the Cube and used it to drain Thanos of his power.
In Thanos' eyes, he had failed to prove himself to Death, so he began to search the universe for more powerful weapons, the Infinity Gems. He briefly allied himself with both Gamora, who he trained as his personal assassin, and Adam Warlock to confront Magus, a future version of Warlock himself. During the fray, Thanos absorbed certain energies of Warlock's Soul Gem.
He gradually acquired the five other gems in existence and combined them to make one synthetic gem. Gamora tried to stop him, but he defeated her. Adam Warlock learned of Thanos' plans, and recruited the Avengers and Captain Marvel to put an end to him. Captain Marvel destroyed the gem, but Thanos captured the Avengers and killed Warlock in a head-to-head battle. Spider-Man teamed up with Lord Chaos and Master Order to rescue the Avengers and revive Warlock. Once alive, Warlock battled Thanos yet again, defeating him and leaving him alive, yet immobilized in stone.
Death came to Thanos and healed him. She further increased his powers and asked for his help. She believed that the balance between life and death had tilted, and needed to be restored. Thanos once again harnessed the gems, and used them to make the Infinity Gauntlet, which his power near-infinite.
The Gauntlet gave Thanos unlimited power over Space, Mind, Soul, Reality, Time, and Power itself, and with a snap of his fingers, Thanos cut down the population of Earth by half. Warlock, Wolverine, Captain America, Silver Surfer, Thor, and virtually every remaining hero and even some villains faced him in an epic battle, which Thanos easily won. After he killed his opposition, more celestial beings faced him, such as Galactus and Kronos. He took them down as well.
He then saw that Death was not yet pleased. In a brief moment of vulnerability, Nebula, who Thanos had contorted for his own pleasure, grabbed the Gauntlet from his hand and willed everything back the way it was. The celestial army, brought back into existence, fought again, but this time against Nebula. Thanos was forced to work with Warlock to disarm her. Warlock donned the Gauntlet and set everything right, scattering the gems.
Thanos was banished by Warlock to an uninhabited planet, where he went to work as a simple farmer.
Such is the story of the Mad Titan up until the end of the Infinity Gauntlet series. So far this looks like the direction that Marvel wants to go film-wise with the introduction of the first two Infinity Gems (possibly three if you believe the theory about Loki's staff, which I do). Marvel obviously isn't keen on recapturing the comic events entirely, but I hopefully they make it as accurate as they can.