It is the basic language used to write web pages.
HTML is a mark up language and not a full-blown programming language so is therefore essentially static in nature.
HTML is parsed by your web browser when a web page downloads and consists of tags (commands to tell the browser how to render the text, where to load in graphics etc. on the web page) as well as the actual text.
HTML4
HTML4 adopted many browser-specific element types and attributes, but at the same time began to try to “clean up” the standard by marking some of them as deprecated, and suggesting they not be used.
HTML5
HTML5 is the 5th revision of the HTML standard and as of December 2011 is still under development.
Its core aims have been to improve the language with support for the latest multimedia while keeping it easily readable by humans and consistently understood by computers and devices (web browsers, parsers, etc.).
XHTML (eXtensible Hypertext Markup Language)
It is a reformulation of HTML 4.0 in XML.
It is a family of current and future DTDs and modules that reproduce, subset, and extend HTML.