HTML (HyperText Markup Language) is a markup language similar to XML that is commonly used to create websites. It was created as an application of SGML, has been extended to create XHTML (eXtensible HTML).
The HTML standards, as well as that for many other web technologies, is maintained by the World Wide Web Consortium (the W3C). The current iteration of the language, HTML4, was introduced in 1997, while the next iteration, HTML5, is currently under active development jointly by the W3C and the Web Hypertext Application Technology Working Group (WHATWG).
The language specification and standards documents are available online at the HTML 4.01 Specification documentation.
Useful tools and references