Oxyporus populinus

This is a key to gilled mushrooms, that is, mushrooms having a definite cap with a fertile surface consisting of gills. The fruiting body usually also has a stem, although that may be lateral or absent (usually, then, the mushroom is growing from wood). You can use this key to identify mushrooms that you find.

Fertile surface usually a layer of vertical tubes, of which the mouths are visible as pores on the underside of the cap or shelf.
Fruiting bodies usually tougher or harder than the "normal" gilled mushrooms, being leathery, corky, or woody. But they can be quite tender while actively growing
Once grown, they do not decay easily, remaining on the substrate for months or years
They often grow on wood, although a few are terrestrial (even those are usually growing on buried wood)
Fruiting body is usually a flat shelf, or hoof-shaped, protruding directly from the substrate, although sometimes it may have a short stalk.
Some forms never grow away from the substrate at all, so that all that is visible of the fruiting body are the pores.
Sometimes the pores are so minute that the fertile surface seems solid, until you look closely

Growing on wood

Sessile cap entirely white to buff
Pores concolorous or yellow or pale grey, sometimes darkening in age to grey or even black
Sometimes with moss or algae growing on the cap (coloring it green)

Fruiting body hard or tough
Usually with moss or algae growing on the initially hairy top

Cap furry, broadly attached to tree
Causing a white rot on hardwoods, usually maple
Fruiting body perennial: larger ones have distinct layers of tubes

Oxyporus populinus     (H. C. F. Schumacher: Fries) Donk