Nothing, really. The term HTML5 video is used to differentiate the playing of a video directly by a browser encountering a web page’s video element, as opposed to being played by a third-party browser plugin via an object or embed element. There is nothing intrinsic about any video that makes it HTML5-ish as opposed to Flash-ish. The distinction is entirely a matter of how the a web page is coded to present the video.
Posted in: HTML5