Explain clickthrough and clickthrough rate.

Clickthroughs, or clicks, refer to the number of times that users click on a banner ad. When the viewer clicks the ad they are transferred to the advertiser's web site or to a special page where they are encouraged to respond in some way to the ad. The clickthrough rate, or click rate, indicates the success of an advertiser in attracting visitors to click on their ad. It is the ratio of ad clicks to ad impressions.  For example, if during one million impressions, there are 20 000 clicks on the banner ad, the clickthrough rate is 2%.