CSS defines HOW HTML elements are to be displayed. Whereas, .skin files are you used to change the look and feel of the web controls on your web pages. For example you can change the back ground of how a textbox looks like.

Styles are normally saved in external .css files. External style sheets enable you to change the appearance and layout of all the pages in a Web site, just by editing one single file!

We can store css and a skin file in the themes folder in your ASP.Net project. This is used to organize your web file in your ASP.net projects