Mushroom is a fungus that has been used in cooking over the years. There are innumerable varieties of the mushroom. It is largely used in the Chinese, Korean, European cuisines and others.
Because of their high water content, mushrooms should be stored in the fridge to keep them as fresh as possible. You can probably get away with stashing them on the counter for a day or so, but if you want to keep them for several days, or even up to a week, place them on a shelf in the fridge.