Cain is famous for two reasons. He was the first human to have human parents (Adam and Eve) and was thus the first human to be born into the world (or visa-versa). He was also the first person to commit murder - he killed his own brother, Abel.
The Life of Cain
Cain was a farmer. While we don't learn of too many other details of Cain's life, we can deduce that he was not a very nice person. Based on Genesis 4, he was obviously underhanded, defiant, and self-centered (not to mention a cold-blooded killer). In the New Testament, John (in 1 John 3) says Cain belonged "to the evil one."
After killing his brother, Cain wandered and eventually built a city in the land of Nod (which may have been anywhere; it's location is unknown ). He was married (we are not given the name of his wife), and he and his wife had a son Enoch.
Points of Interest
What's in a Name?
Despite the notoriety of the name Cain, people still name their children Cain. The name "Cain" ranks in the top 1000 male names in 1994, 1996, and 1997. On a somewhat humorous note, in all 3 of those years "Cain" was more popular than "Darwin".