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The XML FAQ — Frequently-Asked Questions about the Extensible Markup Language
Section 1: Basics
Q 1.8: What is HTML5?
A new version of XHTML for use in interactive and mobile environments.
To enable XHTML to be more widely used in interactive and handheld environments, the W3C has developed a revision of XHTML called HTML5, which adds more presentational elements, better support for multimedia and mobile devices, and has a less rigid document model.
There is an explanation from Elliotte Rusty Harold, and a HTML5 FAQ from the WhatWG.
HTML5 in conjunction with CSS3 is now regarded as the prevailing standard for web development.