XML Reference
From Winamp Developer Wiki
The XML Reference is divided into the following sections:
XML Elements
Use this to define the pictures, colors & fonts you want to use in your skin.
XML GUI Objects
Here you'll find all of the GUI Objects that you can use in your skin.
XML Containing objects
The containing objects help organize your skin. You'll need a container and a layout to display a skin. Basically you'll place all your GUI Objects in one of these.