The defensive line has a few different responsibilities, including matching up with the offensive line, containing the offense's run game, and pressuring the passer. Depending on how the defense sets up, different positions may have different responsibilities.
Popular lineups include the 4-3 defense, which places four linemen up front, and the 3-4 defense, which uses 3 linemen. The 4-3 defense puts two defensive tackles between two defensive ends while the 3-4 defense uses a nose tackle between two defensive ends.
Still, a number of other variations are used, including those that use five linemen. As such, this explanation will focus on the 4-3 and 3-4 defenses for the sake of simplicity. Generally, 4-3 defenses are designed to apply pressure to the quarterback directly through the use of the defensive line.
The nose tackle should be an especially large, strong player who can take up a lot of space, even relative to linemen. In a 3-4 defense, he sets up in the middle of the line where he will often be put against two players: the center and a guard.
Though not necessarily as big as a nose tackle, defensive tackles are still strong and quite large. In a 4-3 defense, defensive tackles set up on the interior of the line, right next to each other. They are generally set up across from the guards on the offensive line and do their best to stop running plays by plugging up the A and B gaps, which are the openings between the center and the guards, and the openings between the guards and tackles respectively.
In addition to stopping the run, many defensive tackles are used to pressure the quarterback. Because they are near the center of the line and do not have much room, they are more prone to use a bull rush to get to the quarterback.
Depending on the defense, defensive end personnel are different. In a 4-3 defense, the defensive ends are usually situated across from offensive tackles, though more to the outside, or across from the tight end. These players are somewhat smaller and are usually faster than other linemen so they can get around blockers to pass rush. Defensive ends in a 3-4 defense, however, are bigger and take on a role resembling that of a defensive tackle in a 4-3 defense.