This is basic stuff. CTR is Click Through Rate (or simply click rate). It's the percentage of people who saw your ad that clicked on it (number of clicks divided by number of impressions). Read my Adwords FAQ for answers to more questions you may have.
CTR actually is the impression divided by the clicks.. Example : Your ads has been impressed on Google search for 100 times, and clicked 50 times, it means your CTR is 100/50 = 2% I hope it helps!
Simple; CTR = Click-through rate = Impressions / Clicks. In terms of Google AdWords, you're going to get a better quality score and pay a lower CPC the higher your CTR is, in fact, it's one of the most important factors in quality scoring.