First you'll want to consider paint types. There are two -- oil or alkyd and latex. Latex paints are more popular because they dry quickly and clean up with soap and water. The next thing you'll want to consider is the finish.
Flat finishes have a soft, even appearance. They mark easily, however, so you'll want to use them in rooms that don't get a lot of traffic. Eggshell and satin finishes have a low luster and are washable, making them a great choice for rooms that get more use. Semi-gloss finishes are the most durable and are perfect for rooms that take a lot of abuse. Glossy finishes are shiny and are typically used on trim work.
Now for brushes. There are basically two types: Natural bristle brushes are used for alkyd or oil-based paints. Synthetic bristle brushes can be used for alkyd paints, but are more often used with latex paints. When selecting take a look at the bristles. They should be split or flagged at the ends and have flexible tips. An angled sash brush is great for cutting in corners or for painting trim.