In the botanical world, a conk is a fruiting body of a bracket fungus that grows on the side of a tree. It is usually shaped like a thick half-circle shelf and is either fleshy or fibrous.
While many are found growing on dead trees, they can be a scourge to living ones also. The wood starts to decay as the hyphae (roots) work their way inside. In addition to causing problems by weakening the tree's structure, it also allows other pests like bark beetles and woodpeckers to further wreak havoc.
You can tell how long they have been growing by counting the rings, much like those on a tree trunk.
Some people like to harvest conks to make art. You can draw on the underside and the picture will stay. The artist's conk or artist's bracket (Ganoderma applanatum) is especially suitable for this purpose.
Other names for a bracket fungus are shelf mushroom and shelf fungus.