Offensive backfield
The offensive backfield is the area of an American football field behind the line of scrimmage. The offensive backfield can also refer to members of offense who begin plays behind the line, typically including any backs on the field, such as the quarterback, halfbacks, and/or fullback.
Play in the backfield
Most running plays begin with a hand-off in the offensive backfield. All kicks and punts must take place in the offensive backfield. If the offensive ball-carrier is tackled in the backfield, the team will lose yards, in that the distance they need to attain for a first down is more than at the beginning of the play.
Rules
There are many rules which differ for play in the offensive backfield as opposed to play in front of the line of scrimmage. The 1906 football rule reforms mandated that the offensive team line up with at least seven players on the line of scrimmage, which are now commonly called "lineman". Therefore, a spokesman maximum of four players are allowed in the offensive backfield. The other players not on the line of scrimmage may be positioned anywhere, but all must be at least 1 yard behind the seven or more players on the line of scrimmage. A forward pass can only be legally made from the offensive backfield.