What's the difference between an overgrip and a replacement grip?
Overgrips: Most players use overgrips instead of replacement grips. Overgrips are thinner and don't have a soft cushion underneath them, instead they rely on the original grip under them for the cushioning effect. They wrap over the top of the original grip and provide different features such as tackiness & sweat absorbtion. The great thing about overgrips is that they are cheaper than replacement grips and you change them regularly to keep your grip feeling as good as new at all times. Overgrips usually come in a 3 pack, 10 pack or 30 pack.
Replacement Grips: Some players prefer to use the original grip that comes with the racquet. Once these are worn you need to change them with a 'replacement grip'. These cost significantly more expensive than overgrips but they do tend to last a lot longer. They also provide cushioning and can offer special features like ridges to help find convenient spots for your fingers. Most replacement grips usually come in a 1 pack.