Fonts can be used on printed documents to really accent your message - unfortunately this is not true on the internet. There are some choices for font families that can be used, but different browsers, operating systems and configurations will cause the fonts to be rendered differently. Here are some examples of the five primary choices and how they appear on your browser.
Specific font choices can be made, but the browser displaying the web page will make a substitution if the font is not available on the system. (For example a Windows computer does not come standard with the Helvetica font, but the Arial or Tahoma font on the PC may be used when a "sans-serif" font is called for.)
Beyond using these generic font families you have the choice of using "pictures of words" to display your message so that every browser will render exactly the same thing. This allows you exact control of the site visually but it does have several disadvantages:
Silverlight (The technology used to produce the banner on this site) allows the font to be embedded so that it will be properly rendered on the target computer.