The Official Google Blog has posted an overview of the Quality Score system used within the AdWords pay-per-click advertising system. It explains in simple terms how and why this system is used and what it means for advertisers, with the underlying issue of relevancy and value of a keyword or advert providing more benefit to the searcher and the advertiser.

As Google states in the post: "The quality score gives search engines a way of aligning the incentives of the buyers, the sellers, and the viewers of ads. The search engine wants to sell ad impressions, but advertisers want to pay for clicks. The solution is for advertisers to bid on a cost-per-click basis, while the search engine estimates the total value of the ad over time: bid per click times the expected number of clicks."

The post finishes by asking why quality scores are important - to which the answer is that "they lead to a better auction by allowing advertisers to buy clicks, publishers to sell impressions, and users to see relevant ads".