The number: Each side has a large roster of members, but there can only be 11 members of each team on the field at any time.
The sides: The team that has the ball is called the offense, because they are trying to make their way up the field to score. The team that is trying to prevent them from scoring is called the defense.
The offense: The types of players on the field depend on whether the team is playing offense or defense. Offensive team members include positions like the quarterback, who throws or runs the ball up the field and calls plays; the running back, who runs the ball after being passed it from the quarterback; or the wide receivers and tight ends, who are the ones who can catch the ball when it's thrown by the quarterback. Other players on the offense try to protect their teammates with the ball from getting tackled or otherwise stopped by the defense.
The defense: When a team is playing defense, they bring out the big guys to try to stop the other team — linebackers, cornerbacks, defensive ends, and safeties, who all play a role in tackling or otherwise stopping the quarterback and other offensive players.
Each team also has a group called special teams, which are responsible for kicking the ball in different situations, like a punt or a field goal.