Firstly, let's take a look at what a gearbox actually is. In the simplest of terms, a gearbox is a mechanical component containing gears. These are rotating units designed to interact in a way that affects the speed, direction and torque of a machine, such as a car or industrial machine. When engaged, the gearbox transfers energy from one gear to another in order to achieve the desired effect.
In mechanics, gearboxes are part of a bigger system called the transmission. The transmission is responsible for transferring power to the machine; i.e. the transmission is responsible for making a machine ‘go'.
These components work together to affect the speed and torque of a vehicle, two things which greatly affect the quality of driving. By transferring the rotational power created by the engine to the wheels, you can control the car when accelerating, deaccelerating, driving around corners, driving up and down different gradients and during any other road conditions.
If the transmission system fails to transfer energy between mechanical components correctly, it will put undue pressure on the internal combustion engine and you will notice shuddering and shaking as you switch gears.