Bradley Wardell has written up a "A complete customization primer" In relation to the skinning of Windows.
Here is a blurb from this 5 page review @ Betanews.
"In the grand scheme of things, there is a single term for all that we are discussing here -- customization. As people have become more comfortable using Windows, their desire to customize it to suit their needs has grown. So how does one customize Windows?
That's where strange terminologies such as Skins, Themes, Packages, Visual Styles, Schemes, etc. come into play. There is no hard and fast rule as to what makes what. But in general the term "Skin" is used to describe a collection of graphics that are used by a program to graphically alter the appearance of an existing graphical user interface (GUI). By contrast, a "theme" is a collection of graphics that are used to customize other parts of the operating system (not the GUI).
As an example:
Changing the scrollbars and title bar of a program would be to skin it. Changing the wallpaper and icons of something would be to apply a theme ..more