Air stones are one of those items that some people say you need them and some people say you do not. I say you need them. There main purpose is to aerate the water. After all, you can buy them for next to nothing and they add to the aesthetics of your tank.
Air stones come in a variety of shapes and sizes. The most common for small tanks (and the cheapest) are the 1 inch stones. These are small enough they can be placed anywhere, even inside of decorations for a cool effect. There are also discs, rods, and flexible rods.