A defensive strategy by a team that places a set of down linemen and a set of secondary defenders behind them. This is considered an agressive style of defense because it requires the two levels to be extremely versatile and athletic. Most NFL teams have a three level defense.
Sometimes referred to as a "Blitzing Defense."
Very few NFL teams stick with a single defensive scheme throughout an entire game. Different parts of the field require changes in defensive posturing. Also, the yardage needed for the offense to get a first down can dictate the defensive package. Defenses will try to show many different set-ups throghout a game to confuse the offense and the quarterback. This is really a game of strategy as each side tries to predict the most effective way to thwart the other's efforts.