Replacement Grip -This is the grip that comes with a new racquet. It's usually alot thicker then an overgrip and comes in mainly black, white and even blue. The thicker replacement grip is designed as the main base grip to overcome vibration and also provide comfort.

Overgrip -Is usually a much thinner grip designed to go over your replacement grip. This is done for a few reasons. The first is to increase grip size and the second main reason is because they are alot cheaper, when worn out they can easily be replaced at a fraction of the cost. Overgrips will also absorb sweat and oils from your hand. So as you can imagine once these overgrips have done their job they can easily be removed and a new fresh one added.