The Dock is a quick launch of various applications. The dock is normally located at the bottom of the screen, but can also be moved to either side of the screen or hidden from view until one moves his/her mouse over the area where the dock is located. If the application that you want to use is located on the dock (rollover the icons to see the names of each application), simply click the icon and the application will open for your use.
Icons with triangles underneath signify that those programs are open and running even if you cannot see the window.