One of the hardest things to measure in basketball is individual perimeter defense. Measuring interior defense is no picnic either, but in general, high shot-block and defensive rebound rates correlate to good overall team efficiency when it comes to preventing points. Teams with good defensive big men tend to be good overall on defense, and teams that lack a defensive anchor tend to give up a lot of points -- not always, but most of the time.
It's not nearly as cut and dry when it comes to the point guards and wing players around the league. Coaching philosophies play a large part in perimeter defense, as does the quality of a team's big men. If you have Dwight Howard playing behind you, you should be able to be more aggressive out on the floor and cut down on open looks. That's theoretical, as this season's Los Angeles Lakers have managed to play bad defense even with Howard manning the middle.