What is HTML 5
HTML5 is a new version of HTML4, XHTML1, and DOM Level 2 HTML addressing many of the issues of those specifications while at the same time enhancing (X)HTML to more adequately address Web applications. Besides defining a markup language that can be written in both HTML (HTML5) and XML (XHTML5) it also defines many APIs that form the basis of the Web architecture.
Some HTML 5 Features :
Source : Google IO 2010 Day 1 Keynote 9min 34s screenshot.