Spammers often include links in their messages as a way to get you to visit a website. An HTML link in a message looks like the following:
<a href="http ://www.ipswitch.com /sla/index"</a>
This may or may not be accompanied by a tag calling an image or graphic. You should be careful when selecting this feature to filter on. Many legitimate HTML email messages contain links in them, and as such would be identified as spam.