There really isnt much difference at all, except for one. Usually the main page of a website is called index. If thats the case, then it's usually a .html extension. I have my entire site stored in a folder(and subfolders) on this computer. The main site has one html file(the index), and the rest are htm. I also have 4 subfolders. 2 of them have entirely htm files and pic's. Another is all pics, and the other folder has its own index.html and some htm's. Other than that, it shouldnt really matter nowadays.
