From HTML WG Wiki
HTML (Hypertext Mark-up Language) has been the World Wide Web's (WWW) mark-up language. Its purpose has been to provide a method to structure information presented on the web.
Over the years, the capability of the web has grown and so has potential for the capability of HTML. HTML5 maintains backwards compatibility with earlier HTML specifications and adds new features to structure information.
This information may include the words of documents, static or animated graphics, multimedia such as video and audio, and data. The structure is how information is organized or configured, such as in chapters and paragraphs, as well as primary content versus secondary content, such as navigation bars or advertisements.
Structure provides visual indications that aid comprehension of the information. HTML5 improves the ability to communicate structure to the impaired aurally and by other means.
The HTML5 specification is written primarily to be clear for implementors of user agents. This web developers guide encourages HTML writing styles that will permit web pages to render now and in the future in HTML5 capable browsers and other user agents. Also provided are methods for graceful backward compatibility of new HTML5 features with HTML4.
Web developers who are familiar with previous versions of HTML should become acquainted withthe differences of HTML5 from HTML4.