Every page has links to information that is considered to be relevant, but that's only half the story. What about all the people you've never heard of that think your page is relevant to theirs and so have linked to you? Backlinks are (strangely enough) links backwords to documents that reference yours. You can search at Alta Vista or InfoSeek for linking pages, by prefixing your URL with the word link:. Alexa supplies a related links service, as do the latest versions of IE and Netscape. Another approach is to use something like Crit which allows visitors to your site to annotate parts of your page by adding LINK tags, these effectively act as a means of backlink discovery, allowing you to see the sites linking to you and what they're saying about your page.