Offense
The defensive team or defense is the team that begins a play from scrimmage not in possession of the ball. The object of the defensive team is to prevent the other team from scoring. The sign that the defensive goal has been accomplished is a 4th down, which usually involves punting the ball.
Unlike the offensive team, there are no formally defined defensive positions. A defensive player may line up anywhere on his side of the line of scrimmage and perform any legal action. However, most sets used in American football include a line composed of defensive ends and defensive tackles and (behind the line) linebackers, cornerbacks, and safeties.
Defensive ends and tackles are collectively called defensive line, while the cornerbacks and safeties are collectively called the secondary, or defensive backs.
Typically, a team will have a safety who also has a reputation of being a hard hitter, as evidenced by Mark Carrier, Rodney Harrison and Bob Sanders, John Lynch, and Sean Taylor to name a slim few. More recently, teams are looking for hybrid safeties who can do both jobs, as in a cover 2 defense, the strong safety has a greater role to play in coverage. Safeties are also used in a variety of blitzes. Defensive back - it is not a specific position, however, it is any position, besides the line, including cornerbacks, safetys, etc., that is behind the line of scrimmage. Typical defensive formations include:

Defensive end (DE)—the two defensive ends play on opposite outside edges of the defensive line. Their function is to attack the passer or stop offensive runs to the outer edges of the line of scrimmage (most often referred to as "containment"). The faster of the two is usually placed on the right side of the defensive line (quarterback's left) because that is a right-handed quarterback's blind side.
Defensive tackle (DT)—(sometimes called a defensive guard), defensive tackles are side-by-side linemen who are between the defensive ends. Their function is to rush the passer (if they can get past the offensive linemen blocking them), and stop running plays directed at the middle of the line of scrimmage. A defensive tackle that lines up directly across from the ball (and therefore, is almost nose-to-nose with the offense's center) is often called a nose tackle or nose guard. The nose tackle is most common in the 3-4 defense and the quarter defense. Most defensive sets have from one to two defensive tackles. Sometimes, but not often, a team will employe three defensive tackles.
Linebacker (LB)—linebackers play behind the defensive line and perform various duties depending on the situation, including rushing the passer, covering receivers, and defending against the run. Most defensive sets have between two and three linebackers. Linebackers are usually divided into four types: strongside (Left- or Right- Outside Linebacker: LOLB or ROLB); middle (MLB); and weakside (LOLB or ROLB). The strongside linebacker usually lines up across from the offense's tight end; he is usually the strongest LB because he must be able to shed lead blockers quickly enough to tackle the running back. The middle linebacker must correctly identify the offense's formations and what adjustments the entire defense must make. Because of this, the middle linebacker is nicknamed the 'quarterback of the defense'. The weakside linebacker is usually the most athletic or fastest linebacker because he usually must defend an open field.
Cornerback (CB)—typically two players that primarily cover the wide receivers. Cornerbacks attempt to prevent successful quarterback passes by either swatting the airborne ball away from the receiver or by catching the pass themselves. In rushing situations, their job is to contain the rusher.
Safety (FS or SS)—the safeties are the last line of defense (farthest from the line of scrimmage) and usually help the corners with deep-pass coverage. The strong safety (SS) is usually the larger and stronger of the two, providing extra protection against run plays by standing somewhere between the free safety and the line of scrimmage. The free safety (FS) is usually the smaller and faster of the two, providing variable and extra pass coverage.
Nickel- and Dime- backs—in certain formations one extra (a fifth) defensive back (called a nickel defense), two extra (a sixth) DB (called a Dime package), or even three extra (a seventh) DB called a Quarter may be used to augment the backfield or defensive line. Nickelbacks, dimebacks, and Defensive Quarterbacks are usually used to defend pass plays with extra receivers, but they can also be used to rush quarterbacks or runningbacks more quickly than linemen or most linebackers can. A starting cornerback who is good at blitzing and tackling will sometimes be referred to as a nickelback to distinguish them from cornerbacks.
Six defensive linemen, two linebackers and three defensive backs (the 6-2 formation)
Five defensive linemen, three linebackers and three defensive backs (the 5-3 formation)
Four defensive linemen, three linebackers and four defensive backs (the 4-3 formation)
Four defensive linemen, four linebackers and three defensive backs (the 4-4 formation)
Three defensive linemen, four linebackers and four defensive backs (the 3-4 formation)
Four defensive linemen, two linebackers and five defensive backs (the Nickel formation)
Four defensive linemen, one linebacker and six defensive backs.(the Dime formation)
One defensive linebacker, three linemen and seven defensive backs (the quarter defense) Special teams

American football strategy
Glossary of American football
Lineman (football)
Defensive back