HTML has been in continuous evolution since it was introduced to the Internet in the early 1990’s. Some features were introduced in specifications; others were introduced in software releases. In some respects, implementations and author practices have converged with each other and with specifications and standards, but in other ways, they continue to diverge.
HTML4 became a W3C Recommendation in 1997. While it continues to serve as a rough guide to many of the core features of HTML, it does not provide enough information to build implementations that interoperate with each other and, more importantly, with a critical mass of deployed content. The same goes for XHTML1, which defines an XML serialization for HTML4, and DOM Level 2 HTML, which defines JavaScript APIs for both HTML and XHTML.
This document describes the differences between HTML4 and HTML5 and provides some of the rationale for the changes. This document may not provide accurate information as the HTML5 specification is still in development. When in doubt, always check the HTML5 specification itself.
A surprisingly readable document, given that it was published by the W3C. An english-language version is available here (combining that HTML 5 document and a few others).
http://www.w3schools.com and http://www.w3.org/TR/html5-diff/ gives good idea about html5 and the changes from html4.