The 11 defensive positions in football are made up of three distinctive groups: the defensive line, linebackers, and defensive backs. The defensive line is made up of the defensive tackles (DT) and defensive ends (DE). Usually, there are four players on the defensive line, two defensive tackles and two defensive ends. Then, there are linebackers. The linebacker group is made up of the middle linebacker (MLB) and the left and right outside linebackers (LOLB, ROLB). Finally, the defensive backs (sometimes called the secondary). The defensive backs are made up of cornerbacks (CB) and safeties (S). The most common defense consists of four defensive linemen, three linebackers, and four defensive backs for a total of 11 players.
When a team is on defense they have two main goals:
The defensive line is a key part of every football team.
The secondary is a group of players that are positioned in the defensive backfield.
Cornerbacks are defensive players in the secondary, but they usually start on the line of scrimmage before protecting against routes in the defensive backfield.
Linebackers are found in the defensive backfield close to the line of scrimmage.
They can serve as an extra pass protector or a blocker, depending on the play. Linebackers can also blitz the quarterback in attempt to get a sack. These are the players responsible for relaying the play calls to the whole defensive unit on the field. There are usually two linebackers on the field, but can be more or less if it's a 3-4 or 4-3 defense.
The free safety and strong safety are defensive secondary players positioned in the defensive backfield. The strong safety is positioned on the strong side of the field, defending against passes and stopping the other team's ball carrier. The free safety is positioned on the opposite side of the other team's tight end on the weak side.
If you want to learn more about the defense of a football team, check out these tutorials: