In WOT, users are who rate the different websites, so it makes the service to be much better than other arguably services like SiteAdvisor, mainly because its database is updated continuously.
WOT is very easy to use, When you search for a certain term through Google, Yahoo, Gmail or Wikipedia you'll see a coloured ring next to the result, the colour of the ring will warn you about that website. Red is dangerous, yellow can be dangerous and green means the website is trusted.
WOT takes into account the privacy, children safety, trustworthiness and vendor reliability and you can rate any website at any time.
Finally, in case you access a website marked as dangerous, it will show a warning advise.
This is the Mozilla Firefox Add-on, you can download the Internet Explorer WOT add-on from the Official Site