The previous version of HTML, HTML 4.01, came in 1999. The internet has changed significantly since then.
HTML5 is intended to subsume not only HTML 4, but also XHTML 1 and DOM Level 2 HTML.
HTML5 is designed to deliver almost everything you want to do online without requiring additional plugins. It does everything from animation to apps, music to movies, and can also be used to build complicated applications that run in your browser.
HTML5 is also cross-platform (it does not care whether you are using a tablet or a smartphone, a netbook, notebook or a Smart TV).
HTML5 can also be used to write web applications that still work when you are not online.
The HTML 5 working group includes AOL, Apple, Google, IBM, Microsoft, Mozilla, Nokia, Opera, and hundreds of other vendors.
HTML5 is still a work in progress. However, all major browsers support many of the new HTML5 elements and APIs.