When I create new pages it ask me to choose between the two. What are the differences, if any.

One has one more letter.

Most servers are configured for each to be the default when a directory is specified. For instance if your site is www.example.com/ and someone types that in the browser, index.htm or index.html would be displayed automatically.

In the past Windows could only handle 3 characters in the extension which is why pages would use .htm. That's no longer the case so you see .html

If I had to choose I would go with .html because it's more common now, but I don't think it makes a different. Either should still work.

Don't think it really makes a difference i use .html because im used to it . and most hosts are compatible with both .htm and .html

It makes no difference at all. I use .htm because I'm lazy.. one less character to type

Oh my :P 1 char less :P

on my dedicated server, .html takes preferance over .htm (although no one at Fasthost can actually tell me why...)
I use .htm to make pages so .html makes a good "coming soon page"

What is the difference between the HTM and HTML extensions?

Short Answer

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.
Long Answer

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.
On most, if not all, servers either file will be sent with a MIME type of text/html by default. This can usually be changed by the server's administrator(s), but is, more often then not, left alone.
On most, if not all, browsers, either file will be displayed as intended (i.e. rendered according to the browser's default manner of displaying HTML documents). This last is due more to the MIME type sent by the server then by the file's extension, but that's a matter for another FAQ.
Generally, the use of htm over html, or vice versa, is left to the author's personal preferences.
Technical Differences

Technically speaking there are few to no important differences. An obvious difference is the addition of the letter "L" in the html extension. The technical difference that the additional letter will make to the operating system is better left to a different discussion but in the context of a web author, the additional "L" will make no difference.
The technical difference that the additional letter will make to an http server (a "web server") is minimal. Usually, a server will use a file's extension to figure out what MIME type to send back to the requesting client. Most servers are configured by default to send back the text/html type when the requested file ends in an htm or html extension. This can be changed by the server's administrator(s) in such a way that one of the above extensions returns a different MIME type then the other, however, this is not a very common practice.
It is a common misconception that a file ending in an htm extension had to have been created on a DOS/Windows 3.x platform. This is because those operating environments limit filenames to a 3 letter extension. However, it is very simple to create a file with a 3 letter extension on most other platforms, as well. Even those that allow longer file extensions.
Default Filenames

The one situation in which there may be a difference between the two extensions is that of a server's default filenames. When a URL that does not specify a filename is requested from a server, such as http://www.domain.dom/dirname/, the server returns a file from the requested URL that matches a default filename. Examples of common default filenames include "index.html," "index.htm," "welcome.html," "welcome.htm," "default.html," "default.htm," etc. However, an administrator can make the server's default filename anything he/she so desires.
In the case of a default file, the author's filename must be exactly the same as the server's default filename. In other words, if your server is configured to use "index.html" as the default filename, your file must be named "index.html" and not "index.htm."
Note that servers are often configured with more then one default filename. Check with your server administrator or ISP for details about your specific server.


Source : http://www.sightspecific.com/~mosh/WWW_FAQ/ext.html

back in the days of DOS and windows 3.1 you could have only 3 letters in a file extension and a maximum of 11 characters, including extension.
Since then with the advent of windows 95 and above, this limitation was removed and extensions have more characters, hence HTM (old) and HTML (new) and HTML is considered the preferred but no actual difference.
Sometimes the sqving of one character is important so HTM.
hope this helps
falcon

Originally Posted by TehChat

Note that servers are often configured with more then one default filename. Check with your server administrator or ISP for details about your specific server.

I think that's really the main thing that most people need to know - that and the "priority" in which the multiple default filenames are given by the server.

On my servers the default page can be index.html, index.htm, or index.php, and in that order.

So for example, if I had the two files index.html and index.php both in the main folder, the default page visitors will see when they go to the domain will be index.html. If I delete index.html, then they will see index.php as the default instead.

Originally Posted by navyfalcon

back in the days of DOS and windows 3.1 you could have only 3 letters in a file extension and a maximum of 11 characters, including extension.

that brings back memories

I think it really doesn't matter, validation is more important.

Further more to my post further up the page, any ideas why index.htm just lands at an apache holding page?? I can only see my sites if I use .html or .php for the fist page

probably index.htm is not in the DirectoryIndex document list

I think that most important thing is that whichever extension you use, you use the same one for all pages in your site.

Personally, I prefer html, it's more common, and you create webpage to be view by others, it's better to stick to the 'norm'.

I haven't used a HTM or HTML extension for quite some time. I use PHP as it parses HTML anyway. However, you do need to be using a PHP enabled server.

What advantages does this give me...?

Well, for example, I develop a static page, pure HTML. I can dynamically generate things like today's date, which date the search engine should come back and which date the page expires.

It just makes life a little easier.

Originally Posted by serandfae

I think that most important thing is that whichever extension you use, you use the same one for all pages in your site.

I agree, otherwise you will end up with many broken links. I prefer .html though.