 <p>My guess is that it is a correlation between which tags are most often used together.</p> <p>For example:</p> <ul> <li>Question A tagged with tag1, tag2</li> <li>Question B tagged with tag1, tag3</li> <li>Question C tagged with tag1, tag2</li> </ul> <p>Then it's natural to assume that tag2 "is related to" tag1.</p> <p>I would say the best place to learn would be <a href="http://my.safaribooksonline.com/9780596529321" rel="nofollow noreferrer">O'Reilly's Programming Collective Intelligence book</a>.</p>