HTML was developed by the W3C until 2004, when members of the HTML working group grew disturbed with the direction the W3C was going with HTML. They felt that the W3C was not paying enough attention to the real-world development needs of the language and focusing too much on XML and XHTML. So they formed a new group called WHATWG (Web Hypertext Application Technology Working Group) devoted to evolving the Web. They started by working on a new specification of HTML - HTML 5.
HTML 5 is a new version of HTML 4.01 and XHTML 1.0 focusing on the needs of Web application developers as well as evolving HTML and addressing issues found in the current specifications.
HTML 5 is the newest specification for HTML, and many browsers are going to start supporting it in the future. One nice thing about HTML 5 is that it attempts to stay backwards compatible. So if you don't want to learn it just yet, you don't need to.
If you build Web applications you will eventually want to learn HTML 5. There are a lot of new attributes and tags built just for Web applications. For instance, there are a number of new event handlers for drag and drop
i hope you understand.and if you want read more
http://webdesign.about.com/od/html5/qt/what_is_html5.htm
you can read more on this refrence link
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