1. Replacement grips

Replacement grips are meant to replace the original (leather or synthetic) grip that your racquet came with. Typically, this requires someone with experience to do this for you. Replacing a grip requires more skill than using overwraps.   

In general, you should replace your grip throughout the year as frequently as you play tennis each week. The recommended replacement grip is listed on the inside throat area of your racket.

Replacement grips types:

Firm is high-density felt, leather, calfskin, or cowhide, and it's the number-one choice of players who want to feel the ball collide with the racquet. The trade-off is comfort, which is where the cushioned grip comes in. It's usually made of a spongy foam that provides relief for your hand, but it makes it tougher to feel where the ball lands on the racquet face.

2. Overgrips

Most racquet grips used to be made of leather and would become hard to grip when the pores became clogged with dirt from your sweat.  A solution to this problem is the use of an overgrip.  Overgrips are "temporary" grips, and it is not recommended that you attempt to actually replace a racquet's existing grip.

Overgrips fit over the existing grip (and thus increase the grip size a bit) and absorb the sweat better. Some overgrips are "tacky" or sticky. Some are even essentially gauze with tape and some sticky powder. They should be replaced about every five times of playing or sooner. Otherwise, they get a little icky. Some overgrips feel rubbery, others cloth like, others a bit powdery.  They are about 5 US dollars for a set of three.

Nowadays there are synthetic grips which are much easier to grip than leather, thus possibly removing the need for overgrips.