Google's advertising program based on cost-per-click pricing.
Click
A click (sometimes called a click-through) occurs when a user sees your ad and clicks on the title of your ad, leading them to your website.
Impression
The "Impr" located on your reporting statistics refers to the number of "impressions" for your ad. The number of impressions is the number of times an ad is displayed on Google or on sites or products in the Google Network.