There is a general natural progression for someone becoming a WinCustomizer. It usually follows this pattern:

Step 1: User changes their wallpaper and is pleased.

Step 2: User decides to change some of their windows icons but finds that they can't easily change that many.

Step 3: User does a google search and eventually finds a customization website.

From there they hear the term "Skin". So what is a skin?

A skin, generally speaking, is a collection of images put together with some sort of script or text file that changes the user interface of a given program or environment. The images are usually in either .BMP format, .TGA format, or .PNG format. Regarldess, they're images. The text file is usually organized using XML or INI to create its own unique format to describe what those images are and where the skinning software should put those images.

There are lots of different programs for skinning and they skin different things. A Winamp skins will change the look and feel of the program Winamp. A SysMetrix skin will change the look and feel of the program SysMetrix.  Other types of skins will change the entire Windows user interface -- all the title bars, push buttons, Start bar, radio buttons, scrollbars, etc. of Windows that aren't skinned by something already.  One such program is WindowBlinds.  Its skins come as .WBA files which are just ZIP files renamed and inside of them is a host of images along with some text files that tell WindowBlinds what should be done with those images.

So there's nothing that complicated or scary about skins. They're just a cool name to describe what is really just a bunch of graphic files put together with a text file to explain what the heck should be done with them. Have fun!