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.