A cutting board is just what the name suggests, a flat surface used for cutting fruits, vegetables, meat and whatnot. But when you go to buy a cutting board at a cookware store you may be faced with a bewildering array of options. The most important consideration should be material and size.
Let's deal with size first, because it's easier. Unless you are going to invest in a maple butcher block table, like I did last year, you are going to want a cutting board that is small enough fit in the sink or dishwasher for washing, but large enough to hold the food you are likely to be cutting up. (Don't put wooden cutting boards in the dishwasher, though, or submerge them in water for any length of time. They'll warp.) Your board should be heavy enough to be stable but not so bulky that you'll have trouble lifting it. About 3 or 4 lbs (1.5-2 kg) is ideal, at least 12" (30 cm) square, preferably a little larger. Actually, they're usually rectangular, which is better. Or you can go oval if you like.
The standard debate in terms of material is wood vs. plastic, and much of this hinges around a claim made some years ago that wood boards were less hygienic because they harboured bacteria. The plastic ones were thought not to have this quality. This debate is, however, not resolved, with strong arguments and "facts" marshalled as "proof" on either side. In general I would advise you to use whichever you like better, just be sure to wash your board well after each use with hot soapy water. You can sanitize it with a light bleach solution if you're worried - 1 tblsp (15 ml) bleach in 1 gallon (1 litre) of water.
You can also buy hard acrylic, glass, and Corian boards, but I think the surface is too hard. They don't absorb the shock impact of the knife, which makes them jarring to use. On the plus side, these boards don't absorb odours or bacteria as easily as wood or plastic ones do.
To rid wood or plastic cutting boards of odours, pour a small pile of salt on the board. Squeeze half a lemon onto the salt, and then use the lemon half to rub the salt into the board. This will remove smells and keep wooden boards light in colour.
Because of the danger of bacterial and odour contamination, it's a good idea to keep different boards on hand: one for really smelly stuff like onions and garlic, one for raw poultry and meat, perhaps one for fish, and one for other foods. Even with my big butcher block I use a small wooden cutting board for the first use (garlic/onion) and a larger one for raw animal flesh.
Wooden boards should be treated with a mineral oil or some such substance (I use beeswax bought at Lee Valley Tools) to build water resistance and reduce the chance of warping. And, like I said, don't submerge them in water or put them in the dishwasher.
Avoid coloured plastic boards because the colour can leach out over time, and who knows what that stuff is. You don't want it in your food.
A final note: some boards have a well - a thin groove - around the outer edge that is used to catch the juice from meats as you carve it. This is useful if you do that kind of thing. If you don't need the well, just flip the board over and use the other side. If you don't have a well but want one, just set the board on a cookie sheet with a rim, and it will work just fine.