Quadrant Model of Reality

FOUR MAIN POSITIONS
https://en.wikipedia.org/wiki/American_football_positions
There are four main positions in this set of players:

Quarterback (QB)
The quarterback is the player who receives the ball from the center to start the play. The most important position on the offensive side, the quarterback is responsible for receiving the play from the coaches on the sideline and communicating the play to the other offensive players in the huddle. The quarterback may need to make changes to the play at the line of scrimmage (known as an "audible"), depending on the defensive alignment. At the start of the play, the quarterback may be lined up in one of three positions. If he is positioned directly in contact with the center, and receives the ball via direct hand-to-hand pass, he is said to be "under center". If he is lined up some distance behind the center, he is said to be "in the shotgun". He can also be in between. This is called a "pistol" formation. Upon receiving the snap, the quarterback has three basic options to advance the ball. He may run the ball himself, he may hand it to another eligible ball carrier to run with it, or he may execute a forward pass to a player downfield.
Running back (RB)
Running backs are players who line up behind the offensive line, who are in position to receive the ball from the quarterback, and execute a rushing play. Anywhere from one to three running backs may be utilized on a play (or even none, a situation typically known as an "empty backfield"). Depending on where they line up, and what role they have, running backs come in several varieties. The "tailback" (or sometimes the "halfback", though this term is somewhat archaic) is often a team's primary ball carrier on rushing plays. He may also catch passes, often acting as a "check-down" or "safety valve" when all other receivers on a pass play are covered. The "fullback" is often larger and stronger than the tailback, and acts primarily as a blocker, though the fullback may also be used for catching passes or for rushing as a tailback does. Fullbacks often line up closer to the line of scrimmage than tailbacks do, so they may block for them. A "wing-back" or a "slot-back" is a term for a running back who lines up behind the line of scrimmage outside the tackle or tight end on the side where positioned. Slot-backs are usually only found in certain offensive alignments, such as the flexbone formation. A similar position, known as the H-back, is actually considered a modification of the normal tight end position (see below).
Wide receiver (WR)

A wide receiver (#87, in white) begins a play in the flanker position
The wide receivers are pass-catching specialists. Their main job is to run pass routes and get open for a pass, although they are occasionally called on to block. Wide receivers generally line up split "wide" near the sidelines at the start of the play. Wide receivers, like running backs, come in different varieties depending on exactly where they line up. A wide receiver who is directly on the line of scrimmage is called a "split end", and is counted among the seven required players on the line of scrimmage. A wide receiver who lines up behind the line (and thus counts as one of the four backs) is called the "flanker". A wide receiver who lines up between the outermost wide receiver and the offensive line is said to be "in the slot" and is called the "slot receiver" or "slotback".
Tight end (TE)
Tight ends play on either side of, and directly next to, the tackles. Tight ends are considered hybrid players, something between a wide receiver and an offensive lineman. Because they play next to the other offensive linemen, they are frequently called on to block, especially on running plays. However, because they are eligible receivers, they may also catch passes. The position known as the H-back is a tight end who lines up behind the line of scrimmage, and is thus counted as one of the four "backs", but otherwise his role is similar to that of other tight ends.
Depending on the style of offense the coaches have designed, the game situation, and the relative skill sets of the players, teams may run formations which contain any number of running backs, wide receivers, and tight ends, so long as the mandated "four backs and seven on the line" rule is followed. For many years, the standard set consisted of the quarterback, two running backs (a tailback/halfback and a fullback), two wide receivers (a flanker and a split end) and a tight end. Modern teams show a wide variety of formations, from a "full house" formation with three running backs, two tight ends, and no wide receivers, to "spread" formations featuring four or five wide receivers, sometimes without any running backs. The I formation is one of the most common.