It depends who you are and where you are! In Britain the word mushroom is usually reserved for members of the genus Agaricus
of which nearly 40 species have been recorded (many of them uncommon and difficult to distinguish). Most mushrooms are edible but the Yellow-staining Mushroom is seriously poisonous. All the rest of the fungi with stems and caps (agarics
) are lumped together as toadstools. The derivation of this word probably has more to do with tod, German for death, than it does with the amphibian.
In North America the word toadstool is not used at all, and all larger fungi (even non-agarics like bracket fungi) are called mushrooms.