There are two kinds of safeties who make life a living hell for their offensive opponents. The strong safety lines up on the strong side of the line and is responsible for for defending against the run game. They key on the tight end or the running back and because of this, they play close in to the line of scrimmage. The free safety plays about 15 or so yards back from the line of scrimmage and is responsible for defending against the pass game. He keys on the quarterback, and can be thought of as football’s version of an outfielder.