HTML
HTML is the language in which The Web is written. And is often used with CSS and JavaScript; which are all usually transported over HTTP.
HTML is a markup language that comes in 2 different flavors. "Normal" HTML, which originally was an SGML-based markup language. And XHTML, which is an XML-based markup language.
HTML is often generated dynamically from a server-side scripting language like PHP.
People who "write" HTML, either by hand or generating it dynamically with a server-side script they wrote, are said to be doing Web Development. (Using a WYSIWYG tool to create HTML, is not considered web development.)
I've been writting HTML markup by since 1995. I contributed to the creation of HTML 5. And contributed to the creation of a number of Microformats.
History
There have been a number of different versions of HTML: HTML 2, HTML 3, HTML 3.2, HTML 4, XHTML 1, and XHTML 1.1, XHTML 2, and HTML 5.
HTML Elements
For a full list of and a references for all the HTML Elements, see my HTML Elements page.
Example HTML
A simple example of an HTML file is...
<html>
<head>
<title>Example Title Here</title>
</head>
<body>
<p>
Hello world!
</p>
</body>
</html>