HTML 5 is the next major revision of HTML (Hypertext Markup Language), the core markup language of the World Wide Web.
HTML 5 is the proposed next standard for both HTML 4.01 and XHTML 1.0, as development on the next version of the latter has stopped. HTML 5 was initially said to become a game-changer in Web application development, making obsolete such plug-in-based rich Internet application (RIA) technologies as Adobe Flash, Microsoft Silverlight, and Sun JavaFX. Such applications would be made obsolete by specifying a standard video codec for all browsers to use. However, in December 2007, the editor of the burgeoning draft specification dropped the recommendation of the free software Theora and Vorbis codecs, after opposition from Apple and Nokia. This means HTML 5 does not currently specify a common video codec for Web development.
The ideas behind HTML 5, originally referred to as Web Applications 1.0, were pioneered in 2004 by the Web Hypertext Application Technology Working Group (WHATWG); HTML 5 incorporates Web Forms 2.0, another WHATWG specification. The HTML 5 specification was adopted as the starting point of the work of the new HTML working group of the W3C in 2007.
Difference between HTML4 and HTML5
Click Here for more.