Cain was the Biblical figure in the book of Genesis, the first son of Adam and Eve. (See Genesis 4 for the full passage)

He was a tiller of the ground, while his younger brother Abel tended the sheep. Apparently God always gave Abel an easier time, and Cain grew jealous. One day when they were in the field, he killed his brother.

Afterwards the Lord asked Cain, "Where is Abel?"
Cain replied "How should I know? Am I supposed to look after my brother?"
Then, according to the Bible, the Lord said:

"Why have you done this terrible thing? You killed your own brother, and his blood flowed onto the ground. Now his blood is calling out for me to punish you. And so, I'll put you under a curse. Because you killed Abel and made his blood run onto the ground, you will never be able to farm the land, it won't produce anything for you. From now on, you'll be without a home, and you'll spend the rest of your life wandering from place to place.
"This punishment is too hard!" Cain said. "You're making me leave my home and live far from you. I will have to wander about without a home, and just anyone could kill me."
"No!" the Lord answered. "Anyone who kills you will be punished seven times worse than I am punishing you." So the Lord put a mark on Cain to warn everyone not to kill him. But Cain had to go far from the Lord and live in the land of Wandering, which is east of Eden.

Cain was the first murderer, and the first to sin after original sin (eating the forbidden fruit). There's an interesting religion called Cainism which involves Cain and hitting bottom (shockingly similiar to Fight Club). I'm sure he died before the Flood.

According to fiction, Cain is supposed to be the legendary father of Vampires, at least in Vampire: The Masquerade.

azzer thinks Gollum from Lord of the Rings shares remarkable similiarities with Cain.