HTML5 is the next major revision of HTML, like previous versions, HTML 4.01 and XHTML 1.1 it is a standard for structuring content while CSS is used to format, style content.
HTML5 is a natural progression for HTML and will further demarcate what is structure and what is style, so some declarations will be dropped as CSS should be used instead of HTML. HTML5 has the aim of, or will be a move towards a more application and media orientated standardisation.
Currently many web applications rely on third party plug-ins to facilitate functionality, such as Adobe’s Flash or Microsofts’s Silverlight. While these have enabled functionality for web sites, developers and end users, they are not standards and as such developers have had to ‘go with’ or hope! that end users will happily download and install the required 3rd party plug-in which ultimately makes a web site / application work for them.
The need for an application and media standard further comes into play with the vast array of diverse devices that are now available on the market for accessing the internet. Some devices, such as the iphone, ipod and ipad range don’t have a plug-in for flash, and the Adobe / Apple fallout has been widely publicised. This only goes to highlight that there is a need for non-proprietary standard.
Some of the new standard incorporates new features such as, but by no means limited to: cross-document messaging, WYSIWYG editable elements, video elements, audio elements, drag and drop, offline web applications and new form features.
The World Wide Web Consortium adopted the HTML5 standard specification back in 2007 and currently it is still in working draft. Estimates by various people for the timing of the specification reaching the point of W3C recommendation range from 2012 – 2022. However many browsers do already support some of the HTML5 stable elements.
Useful links for further reading.
A very informative HTML5 map, with current browser compatibility - http://www.swiss-miss.com/2010/06/what-is-html5.html
Wiki definition - http://en.wikipedia.org/wiki/HTML5
W3C working draft - http://www.w3.org/TR/html5/