A mushroom is the main body of a certain type of fungus. Edible mushrooms include portobellos and shiitakes. Don't mess around with the inedible types.
Walk in the woods after a few days of rain, and you're sure to see mushrooms growing from the damp forest floor. Mushrooms like moist environments, and many mushrooms also grow incredibly fast, which is where the verb form of mushroom, "grow and spread quickly," comes from. The word's origin is ultimately uncertain, although some experts believe it's related to the French mousse, or "moss."