From Wikipedia: HTML5 is a markup language used for structuring and presenting content on the World Wide Web. It is the fifth and current major version of the HTML standard. 

 It was published in October 2014 by the World Wide Web Consortium (W3C) to improve the language with support for the latest multimedia, while keeping it both easily readable by humans and consistently understood by computers and devices such as web browsers, parsers, etc. 

HTML5 is intended to subsume not only HTML 4, but also XHTML 1 and DOM Level 2 HTML.