Your web server is here to answer simple requests coming from a visitor browser. A web browser request a page, which must actually match the name of a file available on your webspace. For instance, all requests for some Joomla-generated content must be done through a single file, called index.php, located in your webspace root directory. This file is the single entry point for your web site.

A request such as : http://mysite.com/index.php?option=com_content&task=view&id=12

Tells your server to call upon the index.php file (and also to pass additionnal instructions - option=com_content&task=view&id=12- to Joomla to allow finding the desired page)

Your web server has no problem with this, because the index.php file actually exists.

Now with SEF URL, a typical page request sent by a user browser will be like : http://mysite.com/my-page-title.html

This obviously should cause a 404 " Page not found" error, as there is no such file as my-page-title.html residing in the root directory of your site.

And here comes the .htaccess file. Its role (well, one of its many possible roles), is to recognize such a situation, and instruct your server not to declare a 404 error, but instead send the whole request to Joomla index.php file, which will take care of it.

When the request is actually passed on to Joomla index.php file, then sh404SEF® kicks in and do further decoding of the request to something Joomla can understand.