The Hypertext Markup Language (HTML) file format is used to create/display web pages.
Its main purpose is to align text, images, or links on a website in a specific way. Web pages with .html or .htm extensions are examples of static web site files. Any server or database technologies require another language on top of HTML to create dynamic features in a web site. HTML files are mere plain text files whose contents follow certain rules.
HTML files are usually viewed using a web browser, but can also be opened with a variety of other programs (i.e., notepad, hex editors, etc).
HTML can trace its development from SGML as a text-based markup language. 
The Extensive Hypertext Markup Language (XHTML) is similar in nature to HTML, but has a stricter XML-based syntax.