The offensive backfield in football is the area of the field behind the line of scrimmage on the side where the offense is. Any player not on the line of scrimmage is considered to be in the offensive backfield. Those players are generally the "backs" like the quarterback, running back, and the fullback.
The running back can either be positioned behind the quarterback or to the side. They receive the ball from the quarterback and then run with it.
Fullbacks are similar to running backs, they line up in the backfield either behind the quarterback or to the side as well. The difference is their primary purpose is to be a blocker.
The positions listed above line up in the offensive backfield, but their positioning changes from play to play. The following are commonly used formations within the offensive backfield.
The quarterback (QB) is directly behind the center (C) and there is one running back (RB) several yards behind them
The quarterback is the only offensive player in the backfield, the running backs are either lined up elsewhere or replaced for another position.
Instead of a normal shotgun, which is about seven yards, the quarterback is lined up four yards behind the line of scrimmage. Behind the quarterback is the running back, who is lined up another three yards behind.
There are three members of the backfield, the quarterback lined up right behind the center, fullback (FB) directly behind the quarterback, and the running back directly behind the fullback.
The only position where the quarterback is not in the backfield. The running back takes the snap instead.
All three members are in the back field, the quarterback is in the shotgun with the fullback on one side and the running back on the other.