html or htm,htm vs. html,What is the difference between the HTM and HTML extensions

Other then the obvious, the letter "L," there's not much of a difference between the two extensions. Most, if not all, web browsers and servers will treat a file with an HTM extension exactly as it would a file with an HTML extension, and vice versa. 

Practical Differences
Practically speaking, there is no difference between the two extensions. Both denote that the file contains HTML. This is really a matter of convention and is not an absolute, but most realize that a file whose extension is htm or html contains HTML.