Offense
Quarterback (QB) — The player who takes the snap from the lineman and either hands off, passes, or runs the ball.
Halfback (HB) — The player who usually lines up behind the QB and takes handoffs on run plays (a.k.a. RB — running back). On pass plays he can either run a route and become a receiver or stay in to help block.
Fullback (FB) — Lines up in front of the HB and looks to block players trying to tackle the HB. Can also catch and block on pass plays.
Wide Receiver (WR) — Receivers line up outside the linemen and look to get open downfield on pass plays. The QB throws them the ball and they can run after they catch it.
Tight End (TE) — Most commonly lines up outside the linemen and can either block on run plays or go out on pass plays.
Slot Receiver — This receiver lines up outside the tackles and is a receiver but lines up inside the farthest WR. These players most often run routes over the middle of the field or use their speed to get deep.
Offensive Linemen (OL) — These players block for the QB and HB. The center (C) is in the middle and snaps the ball to the QB on every play. There are guards on either side of the center (RG and LG) and two tackles (RT and LT) flanking the guards.
Defense
Defensive End (DE) — The defensive end matches up against the other team’s lineman on the outside and is most known for trying to sack the QB on passing plays. You have two DEs on the field in most situations (3-4 and 4-3 defenses).
Defensive Tackle (DT) — The DT plays in the middle of the defensive line, usually closest to the center. He is mostly known for plugging up the middle and is usually one of the biggest players on the team (there’s one DT in a 3-4, two in a 4-3 defense).
Outside Linebacker (OLB) — These LBs line up outside and can cover receivers or blitz on pass plays. They must tackle anything that gets past the line on a run play. You have two OLBs on the field in a 3-4 or a 4-3 defense.
Middle Linebacker (MLB) — This player controls the middle of the field for the defense. He stands behind the defensive tackle and must tackle everything that comes through the middle. You have two MLBs in 3-4 and one in a 4-3 defense.
Cornerback (CB) — These players play outside and must cover the WRs on passing plays.
Free Safety (FS) — This player backs up the cornerbacks and helps give them support in the passing game.
Strong Safety (SS) — This player helps in the passing game but can also be brought towards the line of scrimmage to help in the run game as well.
Special Teams
Kicker (K) — The kicker (or placekicker) is responsible for a team’s field goal or extra-point kicks.
Punter (P) — The punter’s role is to catch the ball from the long snapper and kick it downfield to the opposing team on fourth down to help his team’s field position.
Long Snapper (LS) — The long snapper is a specialized player who snaps the ball to the holder on field goals or to the punter so they can set up to kick it.
Kick Returner (KR) — The kick returner catches the kickoff from the opposing team and tries to return it for as many yards as possible to set his team up with good field position.
Punt Returner (PR) — The punt returner is an elusive-style player who catches punts from the opposing team and works to get return yardage.
Third Down Running Back (3DRB) — The third-down running back is often a specialized back who is strong in the passing game as a route runner or a pass blocker against blitzes.
Kickoff Specialist (KOS): The kickoff specialist’s only job is to kick the ball off to the other team. Often the kicker shares this duty, but teams will sometimes bring in a player with a stronger leg, since accuracy isn’t as important in this role.