In American football, a fullback is a running back who specializes in powerful, physical plays. While a halfback focuses on speed and agility to dart through openings in the defense, a fullback is usually bigger and stronger, built more like a linebacker. The fullback's main job is to block for the halfback or quarterback, clearing a path by driving defensive players out of the way. Picture a battering ram opening a castle gate: that's what a fullback does for a teammate carrying the ball.

Fullbacks also carry the ball themselves when a team needs to gain just a yard or two in a crucial situation, like on the goal line. Their power makes them hard to stop over short distances. They might also catch passes out of the backfield or help protect the quarterback from rushing defenders.

In soccer and some other sports, fullback means something quite different: a defensive player who plays near the sidelines, helping prevent the opposing team from scoring. Soccer fullbacks need speed and stamina to run up and down their side of the field, both defending and sometimes joining the attack.