Each of these buttons have a task. Some may do several tasks depending on what mode your camera is in. The model of your camera will also have an influence on the number and function of these buttons. It seems the more expensive the camera the more buttons it has.
Regardless of the cost or size of your dSLR the first step to discovering what each of these buttons do is your instruction manual. As you go through this article you might want to have your manual opened next to you. As we go down the list you can reference your manual and compare your camera's function with what is discussed here.