Designers Tackling the Web

If you mean why use multiple values like that, that's to ensure that your users will have at least one of the fonts that you pick. For instance, the browser will first look for Times New Roman on the users system. If they have it installed, it will use that one. But, if not it will move on to the next font in your "stack". The last one is a generic font family. "Serif" means if none of the fonts on the list are installed, the browser will still render the element in whatever default serif font the user does have.

This is helpful to get around the fact that different fonts are common on different operating systems (Windows, Mac, and Linux all ship with slightly different fonts). If you search for "font stack", you'll find some lists of the most popular fonts on various operating systems and font stacks that people have put together for maximum compatibility.