Custom navigation menu bar I use the main menu as navigation bar, and I would like to obtain the following result: When the mouse is hovered on one navigation item, the sub menu should be shown as &lt;div&gt; under this item. I would like to be able to style the sub menu &lt;div&gt;s as well. 

Shall I actually use the navigation menu or just the main menu?

I am using Drupal 7.
It sounds to me like you are looking for something like Superfish or Nice Menus.

My personal preference is Superfish, as it has become pretty damn useful in recent updates. It's also responsive if that's a requirement, and support touchscreens quite nicely.

EDIT

Something I missed (possibly another personal preference) is that I find the CSS for Nice Menus an absolute NIGHTMARE in comparison with Superfish.