The defense’s job is to stop the opposition offense from getting into their end zone to score a touchdown or from getting close enough to score a field goal. The image above shows a defensive formation known as the 3-4 due to it having 3 linemen (NT, DE) and 4 linebackers (OLB, ILB).
Defensive Line (NT, DE) – The defensive line is comprised of 3 or 4 big, strong players during most regular plays. In a 3-4 defense there is one Nose Tackle (NT) who’s job is to tie-up offensive linemen so that other players can get free to ‘make a play’ as well as stuff the run plays. The Defensive Ends (DE) are also responsible for stuffing/disrupting the run plays of the offense as well as trying to get to the Quarterback during passing plays to ‘sack’ him.
In a 4-3 defense one of the Inside Linebackers (ILB) is removed and an extra lineman is introduced. This extra lineman lines up next to the (NT) and is known as a Defensive Tackle (DT). There’s no need to worry about this yet as you learn the basics.
Linebackers (OLB, ILB) – Players playing one of the linebacker positions have to be strong, fast and have good vision. They setup, at the start of a play, just behind the defensive linemen. Their role is to add extra run-stopping power against offensive running plays as well as being able to ‘drop into cover’ and cover an offensive receiver on passing plays. A third job of the linebacker is to ‘blitz’ the Quarterback in order to hurry him or sack him.
Cornerback (CB) – The cornerbacks lineup in the widest positions on the defense and are responsible for covering/marking the offense’s wide receivers. The cornerbacks have to be fast and agile in order to keep up with the wide receivers and try to break up the passes the quarterback throws to them.
Safeties (FS, SS) – There are two safeties on the standard defensive formation, one is the Free Safety (FS) and the other is the Strong Safety (SS). The Strong Safety lines up closer to the defensive line than the (FS) and is usually responsible for covering/marking the Tight End (TE) on passing plays as well as helping out, like a linebacker, on running plays.
The Free Safety (FS) is the last line of defense, and is usually faster and more agile than the (SS) so he can get to where he’s needed quickly on any given play.