A Website Reputation Rating Program (or Site Advisor) warns the user what website is good, may be good or bad, or green, yellow, red. These programs are usually installed as an Add-on and is used and displayed while using a search engine (e.g. Google, Yahoo!). The Website Reputation Rating Programs usually rate the site bad from phishing or scamming, viruses or spyware, spam, browser exploits and etc.
The reputation metrics are typically collected from other users who have had dealings with the thing that is being rated. Each user would indicate whether he or she was satisfied or dissatisfied. In the simplest case, the reputation of something is the average rating received from all users who have interacted with it in the past. I recommend WOT, Read more here.