How does the Google Quality Score work? The Official Google Blog explains how Google takes preference for quality score in the display of ads:
The quality score gives search engines a way of aligning the incentives of the buyers, the sellers, and the viewers of ads.
The article goes on to say that ads that don't get many clicks will not be shown. It then ends with the idea behind the quality score:
So why are quality scores important? Answer: they lead to a better auction by allowing advertisers to buy clicks, publishers to sell impressions, and users to see relevant ads
The article is appreciated to an extent. Forum members are a bit upset that there is no new information, that the details about the Quality Score are very basic, that Google is only showing ads that make them money, and that it seems (from many forum members, hence why it's being repeated here) that the article was written by a drunk person.
Forum discussion continues at WebmasterWorld.