Car Transmission - A Transmission transmits or moves the engine's rotation power to the wheels in stages using gears. Transmission gears function similarly to the gears on a ten-speed bicycle . A manual transmission requires the driver to manually select gears, while an automatic transmission selects gears automatically as designed.
(from book : Teach yourself visually car care & maintenance - Ramsey, Dan , https://archive.org/details/teachyourselfvis0000rams_h6w0/page/4/mode/2up )