Google Adsense is one of the many ways online publishers can generate revenue. Google is spicing up its ad offerings by introducing expandable ads. An expandable ad is an advertisement that expands in size as you hover the mouse cursor over it or click on it. It gives a bigger area for the advertisement which allows a user to possibly look into some media (video, sound clips, photos) if he is interested to know more about the particular advertisement.

You can read more about Google Adsense expandable ads at the FAQ section of their help site.