CSS allows you to use any fonts installed on the system. font-family allows you to list many fonts. If the parser doesn't find the first one installed on the system, it moves onto the next one. That's why most font-family definitions cascade down to sans-serif or serif. If the browser doesn't have any of your fonts, it uses the default serif, sans-serif, or monospaced font, etc.
If you're a sucker for typography, unfortunately until CSS3 fonts take off you're going to have to live with serving up all the good fonts to your Mac users, since Mac OS X has a far greater selection of great fonts installed than Windows. I get a little depressed when I load up my sites on Windows and I'm greeted with a pile of rigid, aliased fonts.
