Positive displacement supercharers are constantlly on boost, whereas a centrifugal style supercharger builds up boost as the revs build, similar to a turbocharger. Positive displacement superchargers provide more low down torque and quicker power delivery, and usually pull better times on a quarter mile run than compared to the exact same car with a centrifugal charger.
On the other hand, centrifugal superchargers increase power with revs, so the more revs, the more boost, and the more power. They generally produce higher horsepower over positive displacement but lack the low down punch straight off the line.
Obviously each car and setup is different but that is a general comparison between the two styles of superchargers.