Base grip and overgrip
There is the base grip (replacement grip) — it replaces the factory one on the handle. And there is the overgrip — thin, wound on top and changed often. Most people need an overgrip: it costs pennies, yet keeps your hand dry and secure.
Dry or tacky
Tacky grips give maximum traction — good if your hand does not sweat much. Absorbent (dry) ones are for sweaty palms and hot courts: they wick away moisture instead of slipping. Choose to suit your hand.
When to change
As soon as the grip gets shiny, hardens or starts to slip — change it. It takes two minutes and costs pennies, and control comes back at once. Keep a couple of spares in your bag.