A lot of people tend to be mystified by Null objects, not only because of their name but also because they are invisible and by themselves they have no effect whatsoever. Null objects are just marker with a position, rotation and scale that you can place in your scene either as a 2D or 3D layers. They are represented in After Effects by little red squares with 9 handles that you can use to transform them.
Null objects are used as helper objects to mark or control other visual elements in your composition. You canparent other elements to a Null object to control the movement of the child layers by transforming the parent. This allows you to control groups of visual elements with a single Null object layer.
Null objects are usually used to store position data from motion tracking. Check out my motion tracking tutorial if you want to learnhow to track your scene and apply the data to a Null object. Once you have a Null object that follows some visual element in your scene, you can parent other elements, like explosions and fire, to it and they will also follow the movement of the camera.
Let's have a look at Null objects and how you create and use them in detail.