Traditionally, the fonts used on a website were rendered by the browser using fonts resident upon the browser's computer. The website builder could attempt to specify a particular font but if that font was not available then a substitution would be made. There were a few techniques that could be used to render some content, particularly things like headers, in a special font using various forms of image substitution. Those techniques added complexity and could not be used for all circumstances, e.g., in body text.
CSS3 provides a new way to specify the fonts used to render text. This allows websites to either provide their own fonts, or, more significantly, reference external font libraries (e.g., hosted by Google) and control the fonts used by all CSS3 compliant browsers. All of the current versions of popular browsers, even recent versions of Internet Explorer (Version 9, and later), support this facility. We've begun to use this facility on some of our newly developed websites, including this one.