The composition of the teams

Every team is composed of tree units: offensive unit, defensive unit and special teams. First of all, let’s talk about the defensive team: the unit is composed of 12 players, defensive line-mens, corner back, backers and a safety. Here you can see a schema of a 4-3 formation which is composed of 4 defensive line-mens, 3 linebackers, two corner back and two safety player, this is the normal formation. 

Secondly, there is the offense unit which is basically built with five offensive line-mens, two running backs, four receivers and a quarterback. Here is a schema of a basic offense formation:

Finally, there is the special teams, these units are made with offensive and defensive players and are coming on the field on different moments in the game. The first one is the punt; the punt unit is coming on the field when the offense unit, is unable to reach the touchdown zone or when they are too far for a field goal, there is how it works, the team bring on a punter, to kick the ball as far as he can so the defensive players can have a good position on the field. Here is a schema of a standard punt unit (the punt unit is on blue):

 Next, there is the kickoff unit. This unit is specified in long range kick at the beginning of the game, after half-time and after each touchdowns. The goal of the unit is to kick the ball as far as possible so that the other team starts in offence deep in their zone. The ball is placed on a tee in the middle of the field. The unit is usually organized in a symmetrical way. The kicker is in the center and the remaining players go down the field as fast as they can to tackle the player with the ball. Here is a schema of a standard kickoff formation:

The last special unit is called field-goal. It's job is to kick the ball between the two post at the opposing team's side of the field. Field-goal is used to convert a touchdown and add one more point to the score. The second reason why the field-goal team would be on the field is to kick the ball between the post on a fourth down to make three points. In contrary of the kickoff, someone holds the ball during the field-goal. When the ball is snapped, the «holder» takes the ball and puts it on the ground so that the kicker can kick it.  Here is an example of a field-goal: