Recently Google AdSense Team has introduced new concept in Ads that is, Expandable Ads, a new type of Ad that can appear on your respective web pages. AdSense Team say that these ads are served on both a CPC (cost per click) or CPM (cost per impression) basis. You will generate earnings from CPC when user visits the advertiser’s landing page, earnings would not be generated when user simply clicks to expand the Expandable Ads.
If you want to show expandable ads on your site, then you have to enable image ads and follow these steps, steps to enable image ads and remember that expandable ads are currently available only to specific advertisers located in the U.S.
If you would like to have more information on this, please do visit the Google AdSense Blog Site