Advertisers who show their ads on search engines are billed on the number of times these ads are clicked (not on how many times these ads are shown). CTR is the ratio of the number of times an ad is clicked and the number of times an ad is shown.
Number of clicks / Number of times ad is shown.
In the most simple online advertising system, its more profitable to show an ad which has a higher CTR than the one which has the highest bid.
--Shreejay
