Pizza cutters are a brilliant little tool that you really shouldn't be without. They are inexpensive and more useful (and safer!) than a regular knife for cutting through a thick pizza.
There are two main types of cutters. The first is a wheel-shaped knife which rolls as you move it through the pizza, like the one in the picture above. It's also appropriately called, a pizza wheel.
The other one is a large, curved knife which is rocked back and forth to cut the crust. This is called a mezzaluna. Alot of restaurants use a mezzaluna to cut high quantities of pies quickly.