We've recently launched click-to-play video ads, a new ad format for sites in the AdSense network. Video ads may appear on your site if you're opted into image ads for one of the three ad formats that support video ads: the Medium Rectangle (300 X 250), Large Rectangle (336 X 280), and Square (250 X 250).
When a video ad is served to a page, it will be displayed as a static opening image. Once the user clicks the play button or the opening image, the video will begin playing within the ad space. If the user clicks the display URL at the bottom of the ad, or clicks the ad while it's playing, the user will automatically be taken to the advertiser's website. Users can control the volume and can replay the video if they choose; the video will not automatically replay.