URL aliases
"URL" is an abbreviation of "Uniform Resource Locater." It is a fancy way of saying "my page's address (or name) on the web." It is the "name" by which a browser identifies a page to display. We've all seen advertisements that say "Check us out at example.com." Well, example.com is a URL for the home page of their web site (well, sort of - there is an implied add-on to that, such as index.html).
By default, Drupal calls your content "nodes" and identifies them by their position in your database. So your page on "The History of the Macadamia Nut - Part 1" might be known as "node/167." That's all fine and well for Drupal, because it understands that. But your visitor really doesn't care where the page is in the database; all they want to do is find the page again, or know what the entry in their bookmarks list is.
So Drupal has a feature called "URL Alias" that allows you to provide a more understandable name to the content. As far as browsers, servers, and search engines go, it is totally unnecessary. But for humans, it is nearly mandatory. This is why most consultants tell people to always turn on the Path core module, which supports URL aliasing. (we will mention another module shortly, called Pathauto.)
So, just before you submit that exhaustive treatise on macadamia history, and if you have the Path module enabled (and you will want it), then you'll see a section on the edit page that says "URL path settings." So let's say you want your visitors to see it as http://www.example.com/MacadamiaHistory.html. In the URL field, you enter MacadamiaHistory.htm.
Now, some people will argue that putting that ".html" on the end is not necessary. That's absolutely true. But then it's not necessary to do any of this. A common opinion is that if you want your visitors to see a "normal" name, it should look like a normal WEB name. Web pages have some kind of type, such as ".html" so we should too. But it's your preference.
Oops, forgot to do this before you submitted the page? Not to worry - Drupal to the rescue!
First, visit the page you created. In your browser's address field, you'll see its URL. On the end it will probably say "node/xxx" where xxx is some number. Write down that number. Now go to Administer>>Site building>>URL Aliases. There's an "Add Alias" tab at the top. In the top box (sorry, they seem to have dropped a label in 5.1), enter "node/xxx" from above. In the second box, enter "MacadamiaHistory.html".
Now go back and visit that page and look at your browser's URL field.
Automatic Alias Generation
If your site is going to have lots of content, particularly user-submitted content, you might want to looks at the PathAuto module. Not only will this module automatically generate URL aliases for new content (according to rules you can set up), but can even go back and change aliases in bulk.
Path and clean URLs
Note that the Path module does not require Clean URLs. As an example:
With clean URLs:
Without clean URLs:
Permissions
After you enable the Path module, you need to give the appropriate roles permission to use it in admin/user/permissions. Scroll down to the Path module to enable "administer url aliases" and/or "create url aliases" permissions.
HTML vs HTM Extension
Actually the normal HTML file extension is .html (and .xhtml for XHTML)
.htm is the short, non-standard extension Microsoft created for HTML files.
They mean the same, but the latter is incorrect per original W3C/IETF/RFC/etc standards.
Either way, it's completely unnecessary to include the extension in the path.
When you save an existing page as HTML files using the browser Save As... feature, it will add automatically the extension.
On the other hand, most people don't use an extension at all. But Drupal's URL Alias add function will not add the extension unless you do it yourself.
As for using ".htm" vs ".html" it's really a matter of what you want your users to see. If you want to "de-Drupalize" your site, add the extensions; when you want to look even more "Windows-y" use the "htm."
URL aliases are a mostly cosmetic thing, although they can also add a bit to search engine optimization (SEO).