Sam Winchester is one of the two main characters on Supernatural. He's four years younger than Dean.
When he was six months old, Sam's mother was killed by a demon. Exactly 22 years later, this same demon kills Sam's girlfriend right in front of his eyes.
As they grew up, the brothers spent their childhood moving around and hunting these evil individuals. At first, Sam rebelled against his family's line of business - battling demons - but had no choice but to eventually give in to it.
Dean: Well... this is it.
Sam: This is what?
Dean: Team Free Will. One ex-blood junkie, one dropout with six bucks to his name, and Mr. Comatose over there. It's awesome.
Sam: It's not funny.
Dean: I'm not laughing.
Sam: What are you doing here, Ruby?
Ruby: I should be asking you the same thing.
Sam: I'm working a job.
Ruby: The whole world's about to be engulfed in hellfire, and you're here in Magictown, U.S.A.
Sam: You got something against magic?