Snow peas, also called sugar peas, Mennonite peas, and Chinese pea pods, are baby pea pods which are totally edible, hence their French derived name, mangetout ("eat it all"). The Chinese name apparently comes from their origin in Asia, where they are used for stir frying, and this is indeed a wonderful way to prepare them.
Snow peas are flat and thin with tiny seeds inside; they're usually about 1 to 2 inches (2.5-5 cm) long. If they are larger, they may have a string that runs along their thicker seam; just snap off the tip and pull to remove it. Most people remove the tips before cooking. Snow peas become flabby and rubbery if old, so buy them when as fresh as possible: I prefer them to be firm to the feel; if I can fold the pea without it snapping in half, it's too old for me. They are very perishable and should be eaten within two or three days of purchase. They can be blanched, steamed, sauteed, or of course stir fried; just don't cook them too long, no more than a minute or two. (No mushy peas, please.) Raw or blanched snow peas are very good in salads, particularly with a vinaigrette made with sesame oil.
Interesting side note: the sugar snap pea, another popular variety of edible pod pea, is actually a hybrid of the regular round green pea type pea ("English pea") and the snow pea.