The transmission, also known as a gearbox, is a mechanical device capable of multiplying the torque of an engine in order to reduce the speed or gear of the motor to a slower but more productive output. The transmission, therefore, is used for converting speed into torque, a process that is referred to as gear or speed reduction.
There are many types of transmissions for different uses. Automobiles most commonly use manual, automatic, and semi-automatic transmissions. Transmissions designed specifically for automobiles allow for the option of selecting a gear ratio that slows the speed of the engine and generates a higher torque output. Transmissions are connected to the engine via clutch, enabling the input shaft to revolve at the same RPM (revolutions per minute) as that of the engine.
In automatic transmissions, the torque of the engine is multiplied as it is passing through the torque converter, a fluid coupling. It also passes through several gear ratios. Automatic transmissions usually need to have the fluid and filter changed every 24,000 miles (38,624 km) to prevent transmission failure and typically have a shorter service life than a manual transmission. 
Used mostly in North American vehicles, the automatic transmission comprises a set of gear ranges accompanied by parking prawl. Parking prawl is a feature that locks the transmission's output shaft.
Manual transmissions comprise either a synchronized or unsynchronized system and consist of mostly five-speed gears. The synchronized systems are defined by a set of spur gears that revolve in a synchronized fashion by an operator that achieves engine revs with speed. This type of manual transmission is used in more modern vehicles.
The unsynchronized manual transmission is made up of helical gears that intermesh and a dog clutch, which is used for changing one gear to the next. In this kind of manual transmission, such features as synchromesh boxes, friction cones, and synchro rings are used alongside the dog clutch. Agricultural equipment and heavy-duty trucks usually utilize this type of transmission.
While automatic transmissions are commonly used in automobiles, manual transmissions are mostly found on vehicles made for distribution outside of North America. The appeal of manual transmissions is found in its cheaper and more fuel efficient design.
Other types of transmissions include the continuously variable transmission (CVT), infinitely variable, electric variable, hydrostatic, hydrodynamic, and electric transmissions.
Of these, the CVT is one of the most commonly used. The continuous variable transmission consists of infinite ranges of gear ratios that utilize different speeds to create different ratios.