A camera viewport shows the view from the camera, looking at its target. Camera viewport controls include:
Keyboard shortcut to activate a camera viewport: c
If you press "c" to activate a camera view and you have more than one camera in the scene (and no camera is selected), choose a camera from the Select Camera dialog that appears.
You can also activate camera or spotlight views from the right-click viewport menu (under Views). If there are more than ten cameras or spotlights listed, the last entry in the list is "More Cameras" or "More Lights." Choose this to display the Choose a View dialog, which shows the complete list.