"In American football, each team has eleven players on the field at one time. The specific role that a player takes on the field is called their position. Under the modern rules of American football, teams are allowed unlimited substitutions, that is teams may change any number of players after any play. This has resulted in the development of three "platoons" of players, the offense (the team with the ball, who is trying to score), the defense (the team trying to prevent the other team from scoring, and to take the ball from them), and the special teams (who play in kicking situations). Within those platoons, various specific positions exist depending on what the player's main job is." [American football positions. Wikipedia]
The American football positions diagram example "3-4 Defense" was created using the ConceptDraw PRO diagramming and vector drawing software extended with the Football solution from the Sport area of ConceptDraw Solution Park.