from wikipedia:

In computing, a skin is a custom graphical appearance achieved by the use of a graphical user interface (GUI) that can be applied to specific software and websites to suit the purpose, topic, or tastes of different users. A skin may be associated with themes.

Software that is capable of having a skin applied is referred to as being skinnable, and the process of writing or applying such a skin is known as skinning. Applying a skin changes a piece of software's look and feel — some skins merely make the program more aesthetically pleasing, but others can rearrange elements of the interface, potentially making the program easier to use. Although often used simply as a synonym for skin, the term theme normally refers to less-complex customizations, such as a set of icons and matching colour scheme for an operating system — notably, this is how the term was used in association with Windows 95 and Microsoft Plus! for Windows 95.

http://en.wikipedia.org/wiki/Skin_%28computing%29