Let me start by quoting the Interactive Advertising Bureau (IAB):
We'll just say it: It’s time to improve your mobile advertising. 
In their open letter to advertisers IAB urge marketers to implement the HTML5 standard for their mobile ads. They have to, because the advertising industry still clings to Flash like ivy to a dead tree.
Something had to be done
We decided that the only way to help the industry making the transition from Flash to HTML5 was to develop – from the ground up – an HTML5 VPAID Creative framework. Not only did we do that, we also designated this framework to be our primary VPAID framework, while bridging the gap to Flash by a tiny wrapper component. (and when I say "tiny" I mean "tiny"; 5002 bytes, incurring little or no overhead)
We started from the observation that a VPAID advertisement (or VPAID Ad Unit, as the IAB calls it) is in fact an app. An app that runs inside a well-defined environment, on the computer or mobile device of the viewer, for a limited amount of time. Hence, the possibilities are endless, which is why the IAB suggests adhering to some predefined creative formats.
Grand opening of the candy store
Acknowledging the fact that writing apps is not for everyone, we based our VPAID Creative on our Interactive Timeline, leveraging our existing Timeline Authoring Studio. Thus, it can benefit from time-based animation, transitions, video-in-video, user forms, local storage, etc..
It enables you to do crazy things, like showing live television inside your ad. Or – more sensibly – letting the viewer write a message to a friend, inside the ad. Set up a quiz, offering coupons as a prize. Let the viewer interact with the story line, shape his own adventure... Did I mention the word "engagement"?
Wrapping it up
In order for this to work, the authored content is paired with a VPAID Player and delivered via VAST. This VPAID Player is a lean incarnation of its older sibling the Blue Billywig Player, borrowing many of its omni-platform media capabilities and benefitting from its maturation and ongoing development.
Thus, we have created an alternative to Flash for video advertising, enabling you to author and deliver cross-platform interactive ads without programming a single line of code.