A toadstool might sound like something you'd find a fairy living under, but it's really just a poisonous mushroom.
If you're walking in the woods and see especially colorful, mushrooms, they're probably toadstools. In other words, it's not a good idea to eat them. You can use the terms mushroom and toadstool interchangeably — although most people save toadstool for fairy tales and poisonous fungi; it's not a scientific term. The root is probably just the creative combination of the Middle English tadde, "toad," and stole, "stool."