This fan-shaped mushroom (Pleurotus ostreatus) grows in clusters, which have individual mushrooms ranging from about an inch in length to perhaps a foot in diameter; the smaller specimens are considered the best. In the wild they grow on rotting tree trunks and are usually pale grey or tan, but they are easy to cultivate and now come in designer colours like pink and yellow. Here in Toronto, at least, they are now available fresh year round; buy fresh and avoid canned if at all possible.
To use, rinse lightly. The mushrooms tear easily into strips or chunks, ready to be used in any item where a mushroom would seem good. They're delicious stir fried and in Thai soups like tom yam. These are very nice and easy mushrooms to use, and tasty too. For best results, store in a paper bag for no more than a few days.
There are photos of oyster mushrooms at
http://www.specialtymushroom.net/.