A player on the offensive side of the ball who typically lines up behind or beside the quarterback. The main objective of this player is to run block for the running back, or pass block for the quarterback.
Sporting Charts explains Fullback - FB
Mike Alstott was the fullback for the Tampa Bay Buccaneers from 1996-2007 and revolutionized the position. Alstott was able to run block, pass block, as well as run and catch the football. Alstott made it to 6 pro bowls in his career.