What is HTML?

Computer Desktop Encyclopedia: HTML
(HyperText Markup Language) The document format used on the Web. Web pages are built with HTML tags (codes) embedded in the text. HTML defines the page layout, fonts and graphic elements as well as the hypertext links to other documents on the Web. Each link contains the URL, or address, of a Web page residing on the same server or any server worldwide, hence "World Wide" Web.
HTML 2.0 was defined by the Internet Engineering Task Force (IETF) with a basic set of features, including interactive forms capability. Subsequent versions added more features such as blinking text, custom backgrounds and tables of contents. However, each new version requires agreement on the tags used, and browsers must be modified to implement those tags. See HTML tag.
HTML Itself Is Not a Programming Language
HTML is a markup language (the ML in HTML) that uses a fixed set of markup tags. A markup language can also be thought of as a "presentation language," but it is not a programming language. You cannot "if this-do that" like you can in Java, JavaScript or C++. However, in order to make pages interactive, programming code can be embedded in an HTML page. For example, JavaScript is widely interspersed in Web pages (HTML pages) for that purpose. See JavaScript and VBScript.
HTML was conceived as a simple markup language to render research documents. No one originally envisioned Web pages turning into multimedia extravaganzas. HTML pages have been reworked, jury-rigged and extended into full-blown applications. As a result, the source code behind today's Web pages is often a hideous concoction of tags and scripting. See HTML tag, XML, XHTML and SGML.