Firstly, it is important to understand the different between these two grips.
This is usually the original grip that comes with your racket when you purchase a new one.
When this grip gets worn out or dirty, you can change it with a replacement grip.
These are usually thinner than the grip and it is very common for competitive players to change it every match as it is not as long-lasting.
There are different types of overgrips mainly absorbent overgrip and tacky overgrip.