They also protect the runner, forming an alley that he can run through. The quarterback begins each play. They can do this by tackling the quarterback. To do this, their tacklers have to break through the offensive line and get to the quarterback while he still has the ball. Their tackle of the quarterback is called a sack.
Offense is the team in possession of the ball.
Defense is defending their end zone. In other words, they are the team that does not have the ball. An interception occurs when the quarterback passes the ball and it is caught by the other team, who then gains possession of the ball. No, when the ball changes teams, the offense of one team goes off field and their defense comes in. The new team with the ball sends in its defense and brings out its offense.