Both HTM and HTML are file extensions of HTML files. They are files of a plain text type. There are some differences between HTML and HTM. First of all let's discuss what file extensions are.
File extensions are suitable for operating systems because it is used in identify the type of data file thus they can find a suitable program to read or edit it.
HTML stands for Hyper Text Markup Language. HTML is a markup language which helps in making web pages. They are represented as .htm or.html file extensions. When you make a web page by using HTML files then a .html or .htm will most probably seen in the end of its URL.
It is only used as an alternate extension to .HTML. There are some reasons for this which are discussed below as an example.
There are several operating systems such as the Disk Operating System and Window 3.X. These operating systems are not allowing the use of four-letter extensions. Thus use htm, as an alternative of .html. There are only three-letter extensions used in windows such as ‘.doc’ and ‘.exe’, thus .htm is more suitable here. Therefore, .htm is not used on that server. The .htm extension was commonly used in that period when DOS was used. At present, computers are used which can support big files and wide length of names of file. So a four-letter extension is not a problem.
In internet, when you type a URL without HTM or HTML extensions, web browsers can find the source and show its contents. HTM and HTML are only two of the thousands of types of file extensions. At present, HTML is more popular because it is the most common web page format used.