Read the spec here. The most interesting aspect of HTML 5 to me is this:

HTML 5 will be great step forward, standardizing things like dragging and dropping elements on web pages, in-line editing of text and images on sites and new ways of drawing animations. There’s also support for audio and video playback without plug-ins, a boon for usability and a worrisome sign for Adobe’s Flash, Microsoft’s Silverlight and Apple’s QuickTime. The language will also give a boost to web apps, as there are new controls for storing web data offline on your local machine.

Source.