One of the predominant differences between the older Mac Os’s and OSX is the Dock. The dock itself is very straightforward. Icons in the dock represent documents, applications, and folders on your drive. All applications appear on the left and everything else on the right.
A triangle will appear under the Program icon if the program is currently running. That same triangle will always appear under the "Finder" which is the far left icon. To force quit an application in the dock >>> click and hold the mouse over the icon and choose "Quit"