Small to large, rusty orange spored mushrooms that grow on wood (they are saprophytic), often orange-brown themselves, contrasted with Cortinarius that grows on the ground. They are all bitter tasting, which is another good character, making them inedible (and as a matter of fact some are hallucinogenic.) It is unclear how they fit into the larger picture (except for belonging to the dark spored clades), but some Galerinas have been determined to be some of their closest relatives, so those Galerinas will probably need to be renamed. The gills are always attached, but the type of attachment can be variable. They have dry caps, unlike most Pholiotas which they most resemble and which also have a less colourful spore print colour. May or may not have a partial veil when young, which often vanishes instead of leaving a ring. Not usually hygrophanous.
First, the larger (up to 10cm or more) interesting Gymnopilus.
G. punctifolius - Orange with purple-green cap and yellow-green gills. No veil. Likes old growth forests.
G. luteofolius - Veil. Brick red cap and flesh, bright yellow gills, resembling Tricholomopsis. Sometimes growing from timber.
The smaller (<2.5cm to <5cm) species are harder to differentiate: They are all common.
G. picreus grp (incl. G. rufescens) - the second smallest (<5cm), often darker reddish with a darker stem. No veil. Smooth (hygrophanous?) cap.
Unfortunately, nobody seems to care enough about Gymnopilus to write a specialized full colour book about them.