The Navigator
The entire gamut of Library assets is organized into an asset tree whose folder-style structure and general usage should be familiar from file-system tools like Windows Explorer. When you select a location in the Navigator, the folder name appears on the caption of the active location tab, and its contents are displayed in the neighboring Browser.
 
In the Library panel, when you hover the pointer over the Navigator bar (left), it opens the list of categories in an asset tree.