From InQwiki
Occurs when a web visitor sees a link and decides to click on that link. Usually is measured through web analytics and helps to determine the ROI for a given advertising (AdWords) campaign or search engine optimization (SEO) effort. For example, suppose that you launched an add campaign for a given keyword. When people search this keyword on a search engine, your ad or link may appear. If it does appear, then it counts as an impression. If the web visitor clicks on that link then it is considered a conversion or Click Through. The number of times that a web visitor clicks on the link versus the number of impressions that occured determines the click-through rate.
