With several objects and Location, Rotation, Scaling F-Curves for each, the Graph Editor is likely to get cluttered. You need a way to choose which F-Curves you are seeing. It also helps if you can lock an F-Curve so you don't adjust it when you don't intend to:
In the 3D View window, move the cursor over the Cube and select it with Shift+RMB. Note that in the Graph Editor, you can see that there are now channels displayed on the left for both the Cube and the Camera, as shown in the following screenshot.
On the left side of the Graph Editor, click theLMB on the triangle next to the Location channel of the Camera so it points down as in the following screenshot.
The X Location, Y Location, and Z Location channels appear.
Click the eye symbol next to Z Location with theLMB several times and watch the F-Curve for the Z location as you click. Scrub the Current Frame Indicator back and forth and watch the 3D View window...