Mushrooms - hed - are a common ingredient in quite a number of Thai dishes, and quite a few different varieties of the fungus are used. Perhaps the most commonly used are cloud ear mushrooms, which appear in a number of stir-fries. Also called wood ears or tree ears, the Thai name for them is hed hoo noo, which means "mouse ear mushroom". The Thai name is a good description of these thin wrinkly brown fungi. Like all mushrooms, cloud ears have an earthy taste, although it is not very strong in this variety. Even if you don't consider yourself a mushroom lover, you'll probably like the taste of cloud ears. They provide a unique color and texture to dishes such as stir-fried glass noodles, as well as ginger stir-fries. Cloud ears are always found in abundance in the fresh markets, usually at the same stalls selling ginger.
In thin curries, you will see the straw mushroom (hed fang), which are small and range from almost completely spherical to a more elongated egg shape. Straw mushrooms are rather delicate and don't travel well, and so are almost impossible to find outside of Thailand. In soups, you can try using canned but its better to use a similar fresh mushroom such as oyster mushrooms.
Lastly, shiitake mushrooms (hed hawm) are also used in some dishes, especially recipes derived from Chinese origins. Shiitake mushrooms are almost always purchased dried. They must be soaked in hot water or simmered for a few minutes to soften before using. Shiitake mushrooms have a strong woody taste and a texture unlike just about any other kind of mushroom. Their meaty texture probably explains why shiitake mushrooms are especially popular in vegetarian dishes during the annual vegetarian festival.