Gyroscope:

Definitions:

Torque:  A force that causes rotation is called a torque.  
For example, if a door is at a ninety-degree angle, and 
you want to close it, you don't push the door straight 
along the ninety-degree angle.  Instead, you rotate the 
door.  When you exert the force on the door that causes 
it to rotate, you have exerted a torque on the door.

Angular Momentum:  Linear momentum about an axis.  

How it Works:

If you hang a stationary bicycle wheel from a single point 
along its axis of rotation, the wheel will of course, fall 
down due to the force of gravity.  However, if the wheel 
starts rotating it now has angular momentum along the 
direction of its rotation axis.  The angular momentum of 
the wheel is perpendicular to the angular momentum 
produced by the torque due to gravity, so the torque now 
just changes the direction of the angular momentum.  
This results in the precession of the gyroscope.