From Joomla! Documentation
Revision as of 17:33, 21 January 2008 by JGZimmerle
Impressions refer to the number of times the banner has been displayed, while click-throughs refer to the number of times that users have clicked on the displayed banners. The original content provider on whose page the banner was displayed receives a small payment for each impression and a more substantial payment for each click-through, which will often be used to pay for the costs of providing the content to the users.
If banners are too attention-grabbing, users often get annoyed with them and employ an ad-blocker, which prevents the display of banners.
More info on banners is available on Wikipedia .