OS X El Capitan: Dock
The Dock is located at the bottom of your screen by default. It’s a convenient place to keep items you use frequently. You can add or remove apps and documents, make it larger or smaller, move it to the left or right side of your screen, or even set it to hide when you’re not using it.
To add an item to the Dock, just drag the item and drop it where you want it. Place apps to the left of the line in the Dock, and documents to the right.
To remove an item, just drag it out of the Dock. Removing an item from the Dock doesn’t remove it from your Mac.