It is often used as a parent, which can be used to rotate and transform multiple objects at once.
Null objects can also be used in animation with a Look At. You can set the Camera to look at the null object instead of the actual character so that the camera follows the character but does not move with it. Using the null this way will prevent the camera from moving up and down if a character is jumping.
Placing a Null in the viewer
Place the null anywhere in the scene
Place the null at the origin
You can change the display, control type, and shading of the null on the Misc tab of the parameter editor.
For specific parameter help see the Null node help.