From InQwiki
Determines the percentage of people to clicked on a link (click through) given the number of times people were exposed to that link impressions.
Similar to conversion rate only it is the ratio of the number of times that a web visitor clicked on a link from a search engine results page serp over the number of impressions or times that the link appeared in a search result. The higher the click through rate, the more effective an advertisement or combination of page title and meta description tag seems to be worded.
