Alexa.com ranks websites based on visits from users of its Alexa Toolbar for Internet Explorer and from integrated sidebars in Mozilla and Netscape. Each time someone with the toolbar visits a site, it sends notification to Alexa, who then compile statistics and assumptions of the actual total traffic. Some people use Alexa to see a rough estimate of how popular a website is.

From Wikipedia:

There is some controversy over how representative Alexa’s user base is of typical Internet behavior. If Alexa’s user base is a fair statistical sample of the internet user population (e.g., a random sample of sufficient size), Alexa’s ranking should be quite accurate. In reality, not much is known about the sample and possible sampling biases. A known source of bias is the self-selecting, opt-in nature of Alexa traffic tracking software installation, but the significance of this bias on rankings is not reported.

As you might now understand, it is very easy to fool Alexa and give off the impression that your website is more popular than it actually is. The purpose of this website is to expose who those cheaters are, how they do it, and how to spot it yourself.