HTM and HTML refer to file extensions of HTML files. They are files of a plain text type. HTML stands for Hyper Text Markup Language, which is a markup language used for creating web pages. HTML actually uses markup tags for describing web pages. As file extensions, they are denoted as .htm or .html. If you use HTML files to create your web page, then an .html or .htm will most likely appear at the end of its URL. Here are examples: http://code.google.com/chrome/extensions/samples.html and ‘http://edgewisdom.com/Finance1.htm