Hitwise data for January 2009 shows that Google continue their upwards trend to grab most of the searches carried out in the US. Google are now grabbing 72.09% of all searches in the US which is up 9% over the previous year.
Yahoo have lost 15%, MSN have lost 21% and Ask has also lost 21% [...]

Hal Varian, the Chief Economist at Google explains in a most excellent way, the way that Google AdWords determines ranking factors when you are bidding on keywords.
Hal explains that advertisers want to show relevant ads (most of the time), users want to see relevant ads, and Google wants the best for both advertisers and users.
Hal [...]

Google AdWords is a very advanced and very fair Pay Per Click system. If setup correctly, you can be sure that you are getting the best Return On Investment (ROI) on your Google AdWords Pay Per Click engine due to its Quality Score system.
Quality Score is Google’s way of determining how relevant your ads are [...]