A skin is a component of an avatar in Second Life. In avatars with human forms, they are used in conjunction with a mesh body and a mesh head to create a semi-realistic appearance.
There are two types of skins in Second Life; those meant for mesh bodies and those meant for mesh heads. These are typically different products that must be purchased separately. While the products are often sold by the same creators, many only create skins for heads, matching them with skins for bodies from other creators.