Web Safe Fonts
Browsers can use only the fonts installed on the same computer, so it means that every visitor of your web page needs to have all the fonts you want to use installed in his/her computer. Of course, different people will have different fonts installed, and thus come the need of a standard set of fonts.
Here you will find a list of web safe fonts: http://www.ampsoft.net/webdesign-l/WindowsMacFonts.html