The differences between running backs is based on their:
- size and strength
- position on the football field
A half back is a running back that is positioned close to the quarterback on running plays. They take the hand off on running plays and are faster and smaller than the fullback.
A tailback is a running back similar to a half back. Tailbacks are positioned behind the quarterback forming a T shape on the field.
A fullback is a running back that is positioned close to the half back or tailback at the start of the down. Fullbacks are bigger than normal running backs. The fullback defends the ball carrier from being tackled.
On a running play the running back (half back or tailback) will get the ball from a hand off from the quarterback. The fullback will protect the ball carrier (half back or tailback) from being tackled by defensive players.