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.
To get a really good idea of how HTML5 is changing the web. This is a great collection of sites that are currently coded in HTML5. You will find alot of new stuff to play around.