Start off with the basics. There are 11 players on each team. The team that has the ball is called the offense. The team that is trying to stop them is called the defense. The offense wants to move the ball by running with it or passing it. If they can gain 10 yards within four downs, they get to keep the ball. If they don't the other team gets it. The offense's primary goal is to cross the goalline with the ball. That's called a touchdown.
The offense moves the ball by running or passing. The running backs do the bulk of the running, while the quarterback moves the ball through the air by throwing it to the receivers. However, no matter how skilled the running back, quarterback and receivers are, the offense will not be able to move the ball unless the offensive line can successfully block the defense.
The defense attacks the offense and attempts to tackle the player with the ball. Defensive linemen try to defeat the blocks of the offensive line and find the man with the ball. Linebackers have to run to the football and stop the ball carrier. Defensive backs try to knock the pass down, intercept it or tackle the receiver who catches it.
Offensive players try to defeat the defense by dominating with their blocking or confusing the defense with change of direction. If the offense has bigger, stronger and more experienced players than the defense, it will try to run right over the opposition. However, if the teams are evenly matched in strength, speed and experience, the offense will try to fake the defense out of position. For example, the quarterback may try to throw to the receiver on the right side of the field. However, before he throws the ball he fakes handing off to the running back in order to draw the defense out of position.
Tackling is the key to play successful defense. In order to tackle successfully, the defensive player hits the offensive player with his shoulder around the ballcarriers midsection. The instant he hits the ballcarrier, the tackler drives the player backwards and wraps his arms around him to make sure he does not get free.