Hyperlinks can link to any number of different sources (pdfs, emails, external links, popups). By placing an icon beside each of these types it will take the surprise out of clicking on a hyperlink for the user. The plan is to do this in an intelligent manner so users don’t have to worry about placing a class on each link. This is especially handy if you’re building for CMS users that aren’t HTML savvy…
Direct link: Making Hyperlink Icons with CSS