A camera layer lets you define a particular view, with a certain aspect ratio, within your unlimited canvas.
In camera mode (camera track selected) you can do camera moves, rotations and zooms by using the hand tool. (Move with hand tool, zoom with ctrl+hand tool and rotate with alt+hand tool.) Keyframes is created on the current frame and interpolation between frames are linear.
You can set the name of the camera layer as well as the resolution of the camera by double clicking the camera layer. You can create as many camera layers you like with different resolution, moves etc. Just make sure to select the desired one when rendering.
NOTE: A camera layer is already available by default when creating a new document.