When "Web Sharing" is NOT enabled it takes
/usr/htdocs as the root. When you enable "Web Sharing" it will take the config file at
/etc/apache2/httpd.conf. The directory will be the one specified near the following section on the file.
# DocumentRoot: The directory out of which you will serve your
# documents. By default, all requests are taken from this directory, but
# symbolic links and aliases may be used to point to other locations.
Also you con use your "Sites" directory at
/Users/USERNAME/Sites accessing with