HTML 5 is the next major revision of the HyperText Markup Language. It’s currently being developed as the next standard to present content on the World Wide Web. HTML 5 includes new features that were previously handled by various plugins. These new features include video and sound support, drag and drop, client side database storage, canvas for 2D drawing, geolocation. Google recently launched a site to show some of the new features of HTML5 at www.html5rocks.com, of course use the latest version of Chrome to see it in action.
To check how compatible is your browser to HTML 5, you can check out www.html5test.com. I found it interesting that Google developed a framework for Internet Explorer that allows them (the dumb dumb IEs, that is) to run HTML 5.
A good article/diagram about HTML 5 and how it is supported can be found here.
If you want to see some sites using HTML 5 in action, check them out at http://www.html5gallery.com/.