Short answer is that you enable the one you want to display, and disable the others. If there are multiple cameras enabled, you can combine their views together.
Moving the camera is easy as well. Cameras have a transform just like other game objects. They can easily be moved around using the same techniques that are used to move other objects.
In addition, Unity comes with some standard camera scripts that you can use. For example, Smooth Follow makes the camera follow a target GameObject (you specify which one in the editor.)
There are also a lot of camera control scripts you can look at on the Unity Wiki: