A skin is a file that describes the layout and the controls that will appear in the Graphic User Interface (GUI) of the software. The file format chosen for Blue Cat's Skinning language is xml (eXtended Markup Language, see about XML). You don't need be an XML expert to write a Blue Cat Skin. Everything you should know is in the about XML section. The skinning language concepts is quite close to HTML, and Web designers will find many similarities with web site design. It is a lot simpler though, so that anybody can learn it very easily.
Together with the skin come several graphical resources (images, cursors...) that will be used by the skin components.
More than just changing the appearance of a few GUI controls, you can change the behavior of the entire user interface. You can start with changing the resources of an existing skin (modify bitmaps, add you own) and then also modify the layout of the controls and the type of controls you want to display etc. : more than a skinning tool, Blue Cat's skinning language is a complete GUI design language.