An HTM file is a web page written in Hypertext Markup Language (HTML). It's essentially the same as an HTML file; the only difference is the file extension. HTM files contain text, links, and formatting information that web browsers interpret to display a webpage. They are the fundamental building blocks of the World Wide Web. These files use tags to define the structure and content of the page, including headings, paragraphs, lists, images, and links. Browsers read these tags and render the content accordingly. HTM files can be created and edited using simple text editors or more advanced HTML editors. They are typically served by web servers to users' browsers when they request a specific web page. While HTML is the more common extension, HTM was historically used in systems with limitations on file extension length, particularly older versions of DOS. Today, both extensions are generally interchangeable.