DEFENSE!!!!

The white circles are the defensive players.
The red circles are the offensive players.

Without the defense you will be
1. Disqualified
2.and if not disqualified you will lose because they can get points and you can't stop them.

On a defense there are always 11 players. Any more would result in a foul.

The Defensive Tackle's (DT) and the Defensive End's (DE) 'rush' the Quarterback and try to tackle him or pressure him before he can throw the ball.  During a running play Defensive ends will try to get in the way of the running back by getting in his running lanes and sometimes tackling him.

The Linebacker (LB) is an important player on the field. They fill up the running lanes which the Defensive end's and Defensive Tackle's couldn't. There are two types of linebackers. The Middle Linebacker and the Outside Linebacker's (in the picture it says SLB). These guys can also rush the QB  like DE and DF. They also can go after extra WR where there aren't enough CB to cover.

The Cornerback's (CB) (also says SB in the picture) are  another important player on the football field. They try and stop recievers from getting 'open' and not let them recieve passes for yardage, if they do catch the ball, they are usually responsible for any extra yardage after a wide reciever catches the ball. During Run plays they try to evade the WR and tackle the RB.

The Safeties (S) are responsible for cleaning up any mishaps on the field by other players... they are like the extra players and are used to clean up went wrong during pass plays. They can also rush after the QB like  a DE or DT. But on run plays they try to do what everyone tries to do. Fill the running lanes and crush the HB. There are two types of safeties: the Free Safety (FS) and the Strong Safety (SS). The SS is responsible for the running play and rushing the QB. the FS is trying to makesure no WR can catch the ball.