A forward pass in football is a play where the offensive team throws the ball from behind the line of scrimmage. The ball must be advanced to a spot further down the field from where it was originally thrown.
A forward pass can only be used once per down. If one forward pass is completed to an eligible receiver on a down, they must continue with the ball or use a backward pass from there. If the receiver attempts another forward pass on that down, it results in a penalty flag.
If a player brings the football over the line of scrimmage at any point, he may not complete a forward pass. This includes a play where a player makes a backward pass past the line of scrimmage, then attempts a forward pass. This is still an illegal pass.
Offensive Pass Interference: This penalty occurs if the offensive player receiving the football makes contact with the defensive player before the ball arrives. It is often called if it gave the offensive player an unfair advantage to catch a forward pass.
Defensive Holding: This penalty occurs on a passing play when the defensive player holds onto the offensive player and limits their ability to move.
Often times it is a quarterback throwing to the wide receiver, but it may also be the tight end or running back, who are both thrown forward passes. There are also plays in which another player who isn't the quarterback may throw a pass, but these are rare.
Completion: A completion occurs when an eligible receiver on the offensive team successfully catches a pass.
Eligible Receiver: Only an eligible receiver can legally catch a forward pass for the offensive team. An eligible receiver is any player located in the backfield or the two players on the end of the line of scrimmage.