Files with the .HTM extension contain source code for websites, written in HTML – Hypertext Markup Language. It is a string of markers that format the text and images displayed on the website in a browser. The file is saved as plain text, and so it can be viewed using any text editor.

The .HTM extension is equivalent to the .HTML extension, it is, however, less popular. It is used by website project builder software from the Microsoft family – namely Front Page, Share Point Designer, and Expression Web.