Google has launched a beta test for expandable ads units, a new type of ad that can appear on your pages, and can expand to double the original size of the ad to provide more information to your audience about your product. Expandable ads can also include a streaming video or other content designed to engage users through an interactive experience.
Expandable ads on the Google content network are designed to automatically expand in the direction where the most space is available on the screen. They must be clicked by the user to expand and contract. And like other Google ads, you'll earn based on whether the expandable ads on your site are priced on a cost-per-click (CPC) or cost-per-impression (CPM) basis. Keep in mind that you'll generate earnings from CPC expandable ads when a user visits the advertiser's landing page, and not when a user simply clicks to expand the ad.
More info: Google AdSense Blog