Cameras maintain their own aspect ratios, field of view, and frustrum, and project co-ordinates into a space measured from -1 to 1 in x and y, and 0 to 1 in z. At render time, the camera will be rendering to a Viewport which will translate these parametric co-ordinates into real screen co-ordinates. Obviously it is advisable that the viewport has the same aspect ratio as the camera to avoid distortion (unless you want it!).
Note that a Camera can be attached to a SceneNode, using the method SceneNode.AttachObject. If this is done the Camera will combine it's own position/orientation settings with it's parent SceneNode. This is useful for implementing more complex Camera / object relationships i.e. having a camera attached to a world object.