Mushrooms are the spore-producing reproductive structures of certain kinds of fungi. The four items at the right are mushrooms. Notice that each mushroom has a large cap at the top of a stalk. The mushrooms at the far right and far left in the picture drop their spores from tiny holes, or pores on the caps' lower surfaces. The big blue mushroom and the tiny orange one drop their spores from gills on the caps' lower surfaces.
I picked these mushrooms within five minutes of my door so they are typical of what one might find. From the left in the picture, the first mushroom is a Boletus. I think the small orange one is Hygrophorus miniatus. The large blue one is Lactarius indigo, which is edible and which exudes copious deep indigo-colored latex when injured, and the blackish one on the far right is Strobilomyces floccopus, sometimes called "Old Man of the Woods," and it's also edible I hope you're noticing that mushrooms are gorgeous, and their edibility opens up all kinds of possibilities...!
MORE ABOUT MUSHROOMS
A good general site about mushrooms is at Mushroom-Appreciation.com
On the web a site in Denmark called MycoKey helps you identify 283 genera of fungi. The key provides helpful illustrations.
Since mushrooms are just reproductive structures, you wonder what they are the reproductive structures of. One way to understand that is to review the fungus life cycle.
Microscopic spores fall from the pores and gills of mushrooms such as those pictured above. If environmental conditions are just right, fungal hyphae (HI-fee, the plural of "hypha") emerge from the spore -- like the sprout emerging from a seed. These hyphae form a webby mass of typically white, interwoven, threadlike filaments known collectively as mycelium. Each individual, threadlike filament of the mycelium is known as a hypha (HI-fah), plural hyphae (HI-fee). This mycelium composed of hyphae does the organism's day-to-day work of breaking down and acquiring the fungus's food from humus in the soil, or decaying wood or some other substance. In other words, though usually mycelium isn't even noticed by most people, it's actually the fungus organism's body. Then when conditions are right the mycelium mass forms a budlike structure someplace and from this emerges the mushroom. The mycelium does the fungus's work, and the mushrooms enable to fungus organism to reproduce by producing spores.
No. For example, you wouldn't call the two-inch-across structures illustrated at the left mushrooms -- even though the items shown there are indeed the spore-producing reproductive structures of a fungus, called False Turkey-tails (I wonder why?). These kinds of fungus reproductive structures aren't usually considered to be mushrooms because to be thought of as a "real" mushroom, a fungal reproductive structure is usually thought of as having a mushroom shape, plus they must be more or less fleshy in texture. False Turkey-tails are not mushroom shaped, plus they are far too tough and leathery to eat. Actually, the term "mushroom" has no technical basis, so it's pointless to debate about what is a mushroom and what isn't.
What's the difference between mushrooms and toadstools?
Essentially they're the same thing, except that usually toadstools are thought of as poisonous mushrooms. The word "toadstool" is used only in a very informal and vague sense, so we'll not use it again.
Mushrooms have two especially attractive features making them fun to know:
Since mushrooms are such fun to know, today every good bookstore and library holds several mushroom books filled with gorgeous color plates. Nonetheless, sometimes it's hard, even impossible, to come up with a mushroom identification in which you have confidence. That's unfortunate, since getting the identification right is absolutely necessary if you plan to eat wild mushrooms!
In many habitats mushrooms can appear at any time, especially when it's warm and wet. Overnight they can pop up in grassy lawns. Sometimes they form large circles or partial circles known as "fairy rings." If your lawn was recently a forest, mushrooms from fungi that earlier inhabited the forest's leaf litter and root zone can emerge, relics of a bygone ecosystem. Scan the trunks of large trees.
Just keep your eyes open. Mushrooms are just about everywhere, at least where there's moisture and organic matter...