One of the great pluses Joomla! has amongst other CMS is the great flexibility you get using a Joomla! module inside a Joomla! module position. Thinking that you can put Joomla! modules wherever you want using the Joomla! module positions and publish them only on some of your menu items, is just great!
So, what's a Joomla! module position?
Think at a Joomla! module position as a fixed place on your website where, depending on the page that you are visiting, you can publish certain things such as a login form, a contact form, or a sub-menu for instance.