How to use skin files in asp.net 4.0

Hi,

     I am working with  asp.net4.0 version .Can anyone knows how to work with .Skin file. In my current project I have to use skin files. But I don’t have any experience on that. Currently I am using .CSS file for design stuff. I have searched in Google for this, but I am not able to understand how to code and use this .skin file how It will work. Please help me. If anyone knows this concept please send a example or link

 

thanks

Gopal