A gyroscope is a device used for measuring or maintaining orientation, based on the principle of preserving angular momentum. A typical type of Gyroscope is made by suspending a relatively massive rotor/disc inside three rings called gimbals.
It works on the principle of conservation of angular momentum, and consists of a spinning wheel supported on an axis that is free to move on its own. The spinning wheel, or rotor, is mounted on a pivoting support that allows the rotation around a single axis, called a gimbal. By using two gimbals at a time, one mounted inside the other, the gyroscope gives the rotor three degrees of rotational freedom.
As long as the rotor of the gyroscope is being spun, the gyroscope will keep pointing in the same direction. Gyroscopes are mostly used as a basis for inertial navigation systems, meaning that with the help of three very sensitive accelerometers one can detect the motion of the vehicle on which the gyroscope is mounted, on all three axes.
Let us consider the rotating wheel as shown in the figure above pivoted at the shown point O. When a gravitational torque (mg×a) acts about an axis(torque axis) perpendicular to the axis of spin, it starts processing(rotating) about a third axis(axis of precession) which is perpendicular to both the axes mentioned before.