What is transmission system and purpose of the transmission unit?

The transmission system provides different gear or drive ratios between the engine and drive wheels of an automotive vehicle. 
 
The purpose of transmission system are:

1. It must provide varying gear ratios to vehicle.
2. It must also provide a reverse gear for moving vehicle or car in reverse direction.
3. It must also provide a neutral or a disconnecting arrangement so that the engine can be uncoupled from the wheels of the vehicle or car.