Quote from McAfee:
"With SiteAdvisor software installed, your browser will look a little different than before. We add small site rating icons to your search results as well as a browser button and optional search box. Together, these alert you to potentially risky sites and help you find safer alternatives.
These site ratings are based on tests conducted by McAfee using an army of computers that look for all kinds of threats (detailed below). The result is a guide to Web safety.
The SiteAdvisor technology is free, easy to install and even easier to use. And it doesn't collect any personally identifiable information."