Polyporaceae Family

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

In some genera, the walls between some of the pores may be eroded or absent, so that the fertile surface is composed of plates (sometimes very wandering or maze-like) rather than pores; but in this case the plates are very hard and woody, not like the flexible fleshy plates that characterize the gilled mushrooms

In some cases the fertile surface may erode to the point where the fertile surface is composed of extremely irregular teeth. These species are also considered Polyporaceae

Some polypores are terrestrial, and these may be confused with the Boletaceae. The field character most commonly used to distinguish them is detachability of the pores: in the Boletaceae, the pores can usually be peeled cleanly away from the cap; in the Polyporaceae, the cap and the flesh are continuous with one another, and you can't pull the pores off without tearing the flesh of the cap. But some polypores, like Polyporus radicatus will cleanly separate, so they are also keyed out under the Boletaceae
Another developmental feature is that in the Boletaceae the pores develop before the cap opens; in the Polyporaceae, the pores grow after the cap opens, so you can sometimes catch polypores with a fully developed cap having a solid layer of tissue underneath it. If you find something like that, you have either a Stereum or a young polypore.
Finally, bolete tube walls have divergent trama; polypore tubes do not.