A skin is a set of images and audio clips that replace the default ones used by osu!. Many players use skins to allow for a more customised user experience. Most of the important UI elements and sounds in osu! can be overridden, or skinned.
Skins can be used to further optimize gameplay, since many of them hide certain visual details or offer gameplay elements focused on a certain playstyle, such as specific mod combinations. In addition to that, some skins offer custom menus, which commonly feature characters from various anime and games, or are based around certain design styles, such as minimalism or techno.
Skins can either be created from scratch, or downloaded as an .osk file and imported into the game. To import an .osk file, simply open it with osu!, or drag and drop it into the game. If the downloaded skin file has the .zip or .rar format, extract. The resulting folder, which contains the skin.ini file, can then be moved into the Skins folder of your osu! installation.
In addition to the above, many players, including the high-ranking ones, post own personal skins on their userpages, in YouTube video descriptions, or provide them via a Twitch chat command.