Gearbox is part of power train system which provides controlled application of the power from the engine.
Gearbox that uses gears and gear trains to provide speed and torque conversions from a rotating power source to another device through prop shaft ( RWD or AWD) or half shafts (FWD) to the wheels.
engines need to operate at a relatively high rotational speed, which is inappropriate for starting, stopping, and slower travel. Gearbox reduces the higher engine speed to the slower wheel speed, increasing torque at the same time.
transmission has multiple gear ratios (or simply "gears") with the ability to switch between them as speed varies. This switching may be done manually (by the operator) or automatically.
In motor vehicles, the transmission generally is connected to the engine crankshaft via a flywheel or clutch or fluid coupling, partly because internal combustion engines cannot run below a particular speed. The output of the transmission is transmitted via the driveshaft to one or more differentials, which drives the wheels.
Most cars on the roads have fitted manual transmission however we have as well semi automatic or automatic transmission. Semi automatic gearbox are generally designed using manual transmission "internals", and when used in passenger cars, have synchromesh operated helical constant mesh gear sets. We have still manual gear lever to change the gear but no clutch pedal.
Second type of semi automatic gearbox is as well without clutch pedal but gear lever is same as in automatic transmission car. Gearbox has manual transmission internals and have synchromesh operated by driver . For end user is no much difference between this type and automatic transmission.
Some modern cars have last invented DSG (Dual clutch Gearbox). In very short one clutch operating even gears and second odds gears. During change the gear from 1st to 2nd in same time one clutch is realising and second engaging. That makes shifting quick and smooth.
As the engine has got best efficiency and power just on the one exact own speed the best way is keep running as closed as possible this certain of speed using available ratio of transmission of our car. But some fraction of vehicles has got CVT gearbox (Continuously variable transmission). This gearbox keep running engine on one speed and accelerating just through changing ratio continuously. Something like accelerating without raving. Any way even cars with this kind of transmission offering “set of gears” as in automatic gearbox.
Automatic gearbox in simply worlds is kind of transmission which uses to change gears hydraulic oil. First automatic gearboxes was drive by hydraulic oil pressure. Later on to change gears in this kind of gearbox was used electric and electronic control units.