Typically, the transmission has multiple gear ratios (or simply "gears") and is capable of switching between them as the speed changes, which can be done manually (by the operator) or automatically, and can also provide orientation (forward and Reverse) control, there is also a single ratio transmission that only changes the speed and torque (sometimes the direction) of the motor output.
 
In motor vehicles, the transmission is typically connected to the engine crankshaft via a flywheel or clutch or fluid coupling, in part because the internal combustion engine cannot operate below a certain speed. The output of the transmission is transmitted through the drive shaft to one or more differentials to drive the wheels. Although the differential can also provide gear reduction, its main purpose is to allow the wheels at both ends of the axle to rotate at different speeds (this avoids slippage of the wheel during cornering) because it changes the direction of rotation.
 
Conventional gear/belt drives are speed/torque-adaptive mechanisms, alternative mechanisms include torque converters and power conversion (eg diesel-electric transmissions and hydraulic drive systems), hybrid configurations are also available, automatic transmissions use valve bodies based on speed and The throttle input uses fluid pressure shifting.