Fullback (gridiron football)

Immediately behind the quarterback is the fullback. He has two jobs: if it's a running play, the full back will hit the hole at full speed immediately ahead of the running back and will push any remaining defender out of the hole, making certain it's clear for the running back. If it's a pass play, the fullback's job is to stay near the quarterback and block any defenders that happen to make it past the offensive line. The fullback is a fairly quick and fairly big guy, typically around 6' to 6'2 and weighing around 250 pounds.
Sometimes if a team has a very athletic fullback type, they will put him in motion. This means as the quarterback is calling out signals, before the ball is snapped, the fullback will suddenly stand up and start running. When a player goes in motion he may run to the left or the right, but he may not run towards the defense until the ball is snapped. By putting the fullback in motion, you challenge the defense to decide who’s going to guard him. At the snap of the ball, the fullback will turn towards the defense and either hit a key defender or run a bit to catch a quick pass. If the fullback is put in motion frequently, then he's sometimes called an H-Back. When he's used as an H-Back, the fullback may start out lined up just outside the left tackle as if he were an extra tight end, but then go into motion and play from behind the linemen as if he were a fullback.