HTML has been around for quite some time now and the current version that is regarded as standard is version 4.01. This version first appeared at the end of the last century(!) in 1999, so it's time for a new version: HTML5.

Why do we need a new version of the language of the world-wide web? Mainly because there are too many plugins around that aim to do the same thing, which is showing dynamic or streaming content. The major novelty of HTML5 is thus to provide new elements (tags) that allow authors to include such content on their webpages while knowing that these pages will be shown correctly by the browser: without having to rely on third-party plug-ins such as flash or silverlight.

Of course, we still rely on the browsers being able to deal with HTML5 correctly!

For a quick overview of what (the F) is HTML5, look here: http://www.focus.com/images/view/11905/

For a more detailed overview, there's always wikipedia: http://en.wikipedia.org/wiki/HTML_5

And for the diehards, there's the one and only source: http://dev.w3.org/html5/spec/Overview.html and an overview of the differences with HTML 4.01: http://dev.w3.org/html5/html4-differences/