I read a lot about the differences and uses of a grip and overgrip and the truth is not very clear to me. Someone can explain me, please?
The grip that came with your racquet is commonly called a "replacement grip". When you change these, you remove the old one, and replace it with the new one. An overgrip is sort of a thin version of a replacement grip, usually without any adhesive, and it goes over the original grip. Overgrips are usually designed to be more absorbent than a replacement grip - they usually feel better when your hand is wet, but not as good when your hand is dry. However, there are some overgrips that feel "tacky", like an original grip. Overgrips need to be replaced more often, but they also cost less.
Also there are people who says that the original grip shouldn't be removed, or that only someone professional should change it. Is that true?
I usually recommend that people let me change their replacement grips. I do this for a few reasons:
1: They are not replaced very often, so it's not a big inconvenience to have someone do it for you.
2: The customer is usually in the shop, with their racquet, and it takes me about 90 seconds to change one.
3: You do need to be a little more careful with how you wrap them. Overgrips are very thin, so it doesn't really matter if you mess up a little. But replacement grips will feel very lumpy if you overlap them too much. They will also expose the bare handle if you don't overlap them enough.
However, if you are comfortable doing it yourself, there is no reason not to. Besides, at some point, almost every "professional" learned by doing their own racquets.
My raquet is a Prince grip size 2(my hand is between grip 2 and 3). What is the best option?
If your racquet is a size 2, and your hand is between a 2 and 3, then I would do one of 2 things:
If you have trouble gripping the racquet when you sweat, I would use 1 grip + 1 overgrip.
If you prefer the feel of the original grip, I would have a shop build the handle up 1/2 size. There are heat-shrink sleeves specifically intended for this purpose - it shouldn't cost more than a few dollars to have it done.
There really is no right or wrong - what most important is what feels comfortable to you. Personally, I always use overgrips - they help with sweat, and they are cheap and easy to replace - so you can always keep a fresh grip on your racquet.