Skinning is one of the key features of osu!. It enables players to derive from the original skinning elements to create their own! Skins can vary from being for-fun, themed, PRO, or be nigh impossible to play with.
Main page: Skin Set List
For beatmaps only, if your beatmap skin contains a single element from the listed sets in the Ranking Criteria, it must contain all of the other elements within said skinning set. This only applies to beatmap skins, but other skinners may want to consider those lists as well.
Main page: Skinning Tutorial
The list of skinning elements is split up into these categories: