Center

Lined up in the middle of the offensive line, the center snaps the football between his legs to the quarterback to start each offensive play. He is responsible for telling his fellow offensive linemen who to block.

Cornerback

Cornerbacks line up in the defensive backfield across from wide receivers. In a standard defensive formation, there are two cornerbacks.

Defensive End

Lined up at each outside end of the defensive line, defensive ends try to force their way into the offensive backfield to pressure or sack the quarterback or to stop the running back.

Defensive Tackle

Typically lined up on the defensive line opposite the guards, defensive tackles push into the offensive line to disrupt or stop a play in the opponent’s backfield or try to prevent the offense from gaining yardage on running plays. In a 4-3 defense (four defensive linemen and three linebackers), there are two defensive tackles.

Fullback

Typically the larger of the two running backs lined up behind the quarterback, the fullback often serves as an extra blocker for the halfback/tailback on running plays. He can either run with the ball or catch a forward pass on an offensive play, though fullbacks typically carry the ball when a strong running style is needed, like when the offense only needs to gain a few yards for a first down or to score a touchdown.

H-Back/Slot Back

Similar to a tight end, an H-back is used as an extra blocker on running plays and also can catch passes from the quarterback. Where the tight end lines up at the end of the offensive line, an H-back lines up a few yards behind the offensive line in the “slot” between the end of the line of scrimmage and the nearest wide receiver.

Holder

The player who catches and sets the football on placekick attempts, the holder catches and sets the ball on one end so that the placekicker can kick the ball through the uprights on a field-goal or an extra-point attempt.

Linebacker

Lined up three to five yards behind the defensive linemen, linebackers support the linemen in stopping the runner on rushing plays, drop back into pass coverage on passing plays or they rush the quarterback.

A standard 4-3 defense features three linebackers, each with a unique role.

• The middle or “mike” linebacker, relays the play call from the coaches and tells his teammates where to line up before each play.
• The strong side or “sam” linebacker, lines up opposite the tight end and must be ready to cover him on passing plays or take him on as a blocker on rushing plays.
• The weak side or “will” linebacker lines up opposite the sam linebacker and pursues the plays toward the strong side or contains the play if it is a run toward the side of the offense without the tight end.

Long Snapper

The special teams version of a center, the long snapper excels at accurately sending the ball backwards between his legs to the holder on kick attempts or the punter on fourth downs.

Nickel Back

A defensive formation adopted in situations where the offense is expected to pass. A fifth defensive back is brought onto the field to cover opposing wide receivers, usually at the expense of a linebacker.

Nose Guard/Nose Tackle

Lined up on the defensive line opposite the center, the nose guard pushes into the offensive line to disrupt or stop a play in the opponent’s backfield or tries to keep the offense from gaining yardage on running plays. A nose guard is only used in a 3-4 defense (three defensive linemen and four linebackers).

Offensive Guard

Two guards line up on either side of the center on the offensive line and block oncoming pass rushers on passing plays or try to open running lanes for the running back on rushing plays.

Offensive Tackle

Two tackles line up at each end of the basic offensive line formation outside of the guards. Tackles protect the quarterback on passing plays by blocking the opponent’s pass rushers and try to open lanes for the running backs on rushing plays.

Placekicker

A specialized player who comes onto the field for field goals and extra point attempts and also kickoffs. A team could use one kicker for all situations, or it could have a more accurate placekicker who kicks the ball through the uprights and a stronger-legged kickoff specialist who kicks the ball deep downfield on kickoffs.

Punt/Kick Returner

Typically fast and elusive, these players catch the football after his opponent punts or kicks off and then attempts to run the ball toward his opponent’s end zone. His objective is either to return it all the way to the end zone to score a touchdown, or to set up his team’s offense as close to the opponent’s goal line as possible.

Punter

A special teams player who enters the game on fourth downs to kick the ball to the other team after his team fails to gain enough yards for a first down and is too far away from the goalposts to attempt a field goal.

Quarterback

The leader of an offense, the quarterback huddles his teammates and lets them know which plays to run. During a play, he receives the snap from the center and then tries to advance the football toward his opponent’s end zone by running with the ball, handing it to a running back or completing a forward pass to a receiver.

Running Back

Also called a halfback or a tailback, a running back is typically the primary ball carrier and the faster of the two backs lined up behind the quarterback in a standard formation. He can either run with the ball or catch a forward pass on an offensive play.

Safety

Lined up about 10–15 yards from the line of scrimmage in the defensive backfield, safeties defend against the pass, provide support on rushing plays and occasionally rush into the offensive backfield to disrupt the play.

Standard defenses feature two types of safeties:

• A free safety usually lines up farther away from the line of scrimmage and follows the ball, reacting to what the quarterback does. He is “free” to double cover another player and help the cornerback if needed.
• A strong safety typically lines up closer to the line of scrimmage opposite the “strong side” of the offense — the side on which the tight end lines up. He helps on passing plays and supports his teammates on rushing plays.

Tight End

A tight end lines up on the end of the offensive line outside of the tackle and acts as an extra blocker on running plays or becomes a receiver on passing situations. A standard formation uses one tight end, though some offenses call for two.

Wide Receiver

Known for their speed and ability to catch the ball, wide receivers line up close to the sidelines on either side of the offensive line and run downfield and catch passes from the quarterback. A standard formation calls for two wide receivers, but some offenses use three or four at a time.

Source: NFL.com