The :link pseudo-class applies to links that have not yet been visited. The :visited pseudo-class applies once the link has been visited by the user. (Source:W3C)
<p><a href="http://w3.org">Link to the W3C website</a></p>
a:link{color:green;text-decoration:underline}
a:visited{color:red}
In the example, the link should be green and underlined unless you've already been to the W3C's web site, in which case the link will be red.