Toolbar has links to various administration functions, and is displayed at the top of each Administration Screen. Many Toolbar items expand (flyout) when hovered over to display more information.
Main Navigation menu details each of the administrative functions you can perform. At the bottom of that section is a Collapse menu button that shrinks the menu into a set of icons, or to expands to list them by major function. Within each major function, such as Posts, the sub-menu expands (fly-out) when hovered over, and expands fully if an item clicked.