Overwrap is placed over the original wrap that comes with the racket, replacement is supposed to replace it, but some people who like X-tra thickness use it as an overwrap (me )
Overwrap are generally very thin with little or no cushioning. This has the added benefit of not changing the current feel of the racket. As it is so thin is doesn't reduce the feedback from the racket handle as you play shots. A replacement grip is one with you take of the current grip leaving the wood handle bear. These grips come in lots of different guises. You can get plain grips (which are tacky), ripple grips, one specifically designed to deal with sweaty hands etc. These are usually a lot thicker then overwrap and will change how the racket feels. You can wrap the handle with one or more of these grips to help reduce the shock back to your wrist/elbow. But this has the negative effect of reducing your touch with the racket. It is all down to personal choice. Best to try lots of different grips until you find one which sits comfortable in the hand and doesn't slip.
Yes, replacement grips are meant to replace the original grip or whichever one is on the handle of the racket when the time comes to replace it. These tend to be thicker and heavier than overgrips but offer more cushioning. Some people make the racket handle seem thicker by wrapping a replacement grip over the top of the original grip. Overgrips are meant to be wrapped over the top of the original grip. They are thin and light but don't offer as much cushioning as replacement grips. Some people have been known to wrap them of the bare handle. Most people will advise you against this though as it can lead to you having sore hands and elbows because overgrips aren't especially good at dampening vibrations. You can also use more than one overgrip. For example, just two overgrips on the bare handle or a replacement grip and two overgrips. The Karakal PU Super grip is different to the categories above in the sense that really, it can serve well as a relatively thin replacement grip or a fairly thick overgrip. I personally use it as a thin replacement grip. My advice to you is to play around with a lot of different grip setups until you find one that you like. Once you find the setup you like, use it on all of your rackets all of the time.
