HTML5 is a new version of HTML and XHTML. The HTML5 draft specification defines a single language that can be written in HTML and XML. It attempts to solve issues found in previous iterations of HTML and addresses the needs of Web Applications, an area previously not adequately covered by HTML.
Here are several resources worth exploring:
Introduction to HTML 5
Why HTML5 matters – to consumers as well as developers
HTML 5: Features you want desperately but still can’t use