The Mac OS X Mountain Lion Dock
Take a minute to look at the row of icons at the bottom of your display in OS X Mountain Lion. That row, good friend, is the Dock, and those individual pictures are known as icons.
Dock and Launchpad icons are odd ducks; you activate one with a single click. Most other icons in the Finder are selected (highlighted) when you single-click and are opened only when you double-click them. So Dock icons are kind of like links on a web page; you need only a single click to open them.
Here’s the rundown on what happens when you click Dock icons:
If it’s an application icon, the application opens and becomes active. If the application is already open, it becomes active, which brings it and all its windows to the front.
If it’s a document icon, that document opens in its appropriate application, which becomes the active application. If that application is already open, it becomes the active application with this document in the front.
If it’s a folder or disk icon, a stack with its contents appears so you can pick an item. If you choose Show in Finder from this menu, the folder’s window opens in the Finder.
If the item is open already when you click its Dock icon, it becomes active.