Google introduced expandable ads in the year 2009, a new type of ads that appear in your page. These are rich media ads which expands from its original size following user action like Hover. The ads feature YouTube videos like a movie trailer, interactive ads or items for sale etc.
Publishers earn based on whether the expandable ads on the 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.
I recently came across this expandable ad by Quikr,
when you hover the ad appear as
The ad includes a close button and it is more interactive. Publishers can use your Competitive Ad Filter to block contextually-targeted expandable ads.