HTML stands for Hypertext Markup Language. It is the language of the World Wide Web. It is not so much a programming language, but a tag based markup language. That means that it holds the text you see on a page and uses tags to describe the usage of the text.
A basic page would look something like the following:
<html> <body> <h1>This is the page header</h1> <p>This is the first paragraph.</p> <p>This is the second paragraph.</p> </body> </html>
Note how it uses pairs of tags to start the HTML page and end it. It also has a pair of body tags in which the content of the page goes. The actual content is wrapped in special tags too. In this case, h1 is used which tells the browser that the text is a header, h1 being the first and mostly largest header on a page.
There are also paragraph tags for each paragraph. These tags tell the browser how to handle the content. The look (not content) of an HTML page is created using “styles” which are kept in a CSS (Cascading Style Sheet) file.
HTML documents mostly end in .htm or .html. This tells the web server how to feed your browser the document data. In this case it needs to provide the data as it is written in the file without any changes. Dynamic HTML files might have other suffixes and need processing before serving them.
Sir Tim Berners-Lee specified HTML and wrote the first browser and server software in 1990.