HTML is an abbreviation that stands for HyperText Markup Language. It is the language of web pages—the markup language that browsers read to render web pages.

HTML refers to a markup language that has several versions. And web browsers and other user agents have varying levels of support for these versions. In general, the earlier a version is, the more browser support there is for that version.

What is HTML?

HTML is made up of elements (often called tags) that build the contents of a web page. The differences between HTML and other programming languages include:

HTML is not compiled. It is written and used without any changes being done to it. I starts out a text file, and is still a text file when a browser or user agent interprets it. HTML is human readable. While some other programing languages can be read by people (and not just machines), many times you have to learn the language to really understand it.

"Courtesy of about.com"