Most cats don't need to be groomed however there are some circumstances that cat owners will clip or shave their cats. Longhaired cat are sometimes clipped because they get matted or get feces caught in their hair.
If you are looking for a good pair of clippers, choose one that is durable. Here is a pair that I've had very good luck with. It is made by Oster and is a very good quality hair clipper
These clippers are not the cheapest but I've had good luck with it. The cordless ones are also nice and convenient as you can more easily take them to your pet.
An article that might also be helpful to you is Grooming Your Cat