A mushroom is any organism in class Basidiomycota, of the Fungi kingdom (which isn't an SCA Kingdom. Wouldn't it be funny if it was though? Imagine what the populace would be like!). They are characterised by an umbrella shaped cup borne on a stalk.
Some are edible, some are hallucinogenic, others can poison or kill you. The edible ones make excellent ingredients in soups or pies/pasties, or on their own roasted, grilled, marinated in red wine or just raw.