Someone wants to add .css files to my server. I am not exactly what they do. Are they like cgi or php? Can they put code in these type of files to make anything changes on my server? Are there any kind of security risk?

CSS isn't a coding language, it is just a basic principal of dHTML. It is of no security threat to any server, and all free servers allow it :). It's just coding that allows you to more easily maintain a basic "style" (of fonts and positioning) of objects and text. It's actually quite intresting. For more information go to Mulder's Guide to CSS at webmonkey.com.

I don't think they pose any sort of security risk. They are used by web browsers for "style" settings. Font settings etc. Not the best explination but I have a head ache.

CSS can be compared to Javascript in a sense as they both are prettymuch useless without html, but they are both designed to enhance the html experience.

CSS allows you to change the font, color, size or position of all of your text on your website (even if you have 10,000 pages) while only needing to update one file.

Most browsers now support CSS and to my knowledge it will not create a security risk to the user or server.