What is a skin?
A skin is basically the way the background looks on a certain program. You may have used custom skins in various other programs, and now you can in RoK too.
The beta2 client comes with about 10 different skin choices and is set up to easily allow the creation and addition of new ones at any time.
The addition of "skins" to RoK basically just allows you to customize the way your client looks. You can have certain themes or just a certain color or design that you like. Pretty much any kind of picture you can find can be used as part of your skin.