Which are the differences in the bookmarks organization?

In Safari I can organize bookmarks in three group: Bookmarks Bar, Bookmarks Menu and the "Bookmarks" list under the first ones.

I've understood only that the Bar shows bookmarks under the address bar, in spite of the Menu, but no more. So, why Safari separates two group of bookmarks? Just to let the first appear in the main page under the address bar?
And, for example, which is the difference between the Menu and the list? Why are they separated? I can add a bookmarks in the menu but it doesn't appear in the list, and also if I add it in the list it won't appear in the Menu, so what kind of library is it? Shouldn't it have a main group in which having various folders (as Music and playlist in iTunes)? In Safari seems to be these separated groups, like the bookmarks of the list don't take part of the Bookmarks Menu...