There is no difference really. The only concern is that you cannot use them interchangeably in file paths. In other words, if you are linking to a file called "mypage.htm" but the actual file is called "mypage.html", the link won't work. The server will treat mypage.htm and mypage.html as two separate files. Also, some servers are configured to treat "index.htm" as the default page, while others treat "index.html" as the default page. In some rare instances, they will instead treat other files like "index.php" or "default.html" as the default page.