OS X El Capitan: Finder sidebar
Finder windows have a sidebar with items you frequently use, including folders, disks, and tags.
By default, the sidebar is divided into four sections.
Includes items you use most frequently, such as AirDrop, the Applications folder, your Documents folder, and iCloud Drive. You can add other folders by dragging them into the Favorites section.
Displays icons that represent your computer’s disks, any disks attached to your Mac, and any available optical drives.
Shared computers and servers on your local network are listed here, including AirPort devices and Time Capsules. If there are too many shared computers and servers to fit in the sidebar, click All to see the complete list.
If you don’t see a Shared section, there aren’t any servers detected on your local network.
Provides quick access to all the items using a particular tag. Click a tag to see files using that tag.
At any time, you can customize the Finder sidebar. In the Finder, choose Finder > Preferences, then click Sidebar or Tags.