what is a gearbox and how it works?

Answer 1

Check out these two explanations

http://auto.howstuffworks.com/transmission.htm

http://auto.howstuffworks.com/automatic-transmission.htm

Answer 2

gearbox is just another term for Transmission. The transmission transmits the torque (or rotational force) of the engine to the drive shaft(s), Axle(s) and wheels. There are different gears with in the gearbox that allow the vehicle to start from a standstill (low-which gives the most torque output) up through sometimes as many as six gears on some vehicles maybe more on large trucks, (high- which is generally a straight though gear or one-to-one ratio) for highway speeds. Some gearboxes have an over-drive gear which actually increases the output speed of the transmission as compared to the engine speed. which is usually a fuel saver.
A gearbox can also refer to a piece of equipment in industry that transmits the torque of a motor to the drive of a piece of machinery at a specific ratio. (It is still a transmission of sorts).