HTML5 is the future standard for the coding of web pages. Parts of it are useful today.
HTML5 includes a handful of new structural elements that are designed to make markup more meaningful. You can use these elements today; they don't really do much, so browsers don't need to explicitly support them. And it takes only a little trickery to make them work even in IE.
At its purest, the HTML5 video tag is a very simple. To deliver video that plays in Firefox and Safari, however, requires two different video formats, and you still need Flash for IE. Here's the code to make it happen.
Video on the web is a mess. Web standards have never fully embraced video. Until HTML5, there was no video element, so the only way to play video was to depend on platform-specific software. HTML5 provides a video element, but you're going to need to provide video in multiple formats.
HTML5 has been receiving an extraordinary amount of attention, thanks in part to active support and promotion by Google, Apple, and Mozilla, among others. Despite its high profile, however, the HTML5 specification isn't even complete, much less officially blessed or broadly supported by browsers, and there are only a few pieces that are of immediate practical value.