Null object layers

To assign a parent layer, but keep that layer from being a visible element in your project, use a null object. A null object is an invisible layer that has all the properties of a visible layer so that it can be a parent to any layer in the composition. Adjust and animate a null object as you would any other layer. You use the same commands to modify settings for a null object that you use for a solid-color layer (Layer > Solid Settings).
You can apply Expression Controls effects to null objects and then use the null object as a control layer for effects and animations in other layers. For example, when working with a camera or light layer, create a null object layer and use an expression to link the Point Of Interest property of the camera or light to the Position property of the null layer. Then, you can animate the Point Of Interest property by moving the null object. It is often easier to select and see a null object than it is to select and see the point of interest.

A composition can contain any number of null objects. A null object is visible only in the Composition and Layer panels and appears in the Composition panel as a rectangular outline with layer handles. Effects are not visible on null objects.

 To create a null object, select the Timeline or Composition panel and choose Layer > New > Null Object.
Note: The anchor point of a new null object layer appears in the upper-left corner of the layer, and the layer is anchored in the center of the composition at its anchor point. Change the anchor point as you would for any other layer.
If a null object is visually distracting in your composition frame, consider dragging it out of the frame, onto the pasteboard.

Guy Chen provides a simple project on the After Effects Exchange on the Adobe website that demonstrates the animation of several 3D layers arranged as a cube, controlled by a parent null layer.

Angie Taylor provides an extensive discussion and explanation of animation using parenting, expressions, and null object layers in a PDF excerpt from her book Creative After Effects 7: Workflow Techniques for Animation, Visual Effects, and Motion Graphics.