An automatic transmission works by using a system of planetary gears that lock and unlock to accomplish gear changes. Automatic transmissions use a torque converter, rather than a clutch, to disconnect the transmission from the engine when the car is coming to a stop.
The standard automatic transmission found in cars is a hydraulic automatic transmission. The hydraulic transmission contains a planetary gearset, bands to lock the gearset, wet-plate clutches, a hydraulic system and a gear pump. The planetary gearset is what makes it so that the car can shift in and out of different gears, and there are two folded into one gearset inside an automatic transmission.
The planetary gearset has three components: the sun gear, the planetary gears and the ring gear. The gearset works by moving two of these components and keeping another one still. This transmits the torque that the engine makes to the wheels of the car making it so that the car can move. Transmission fluid, stored in a sump under the transmission when not in use, is pulled up by the gear pump into the transmission. The gear pump only has oil pressure when the car is running, which is why an automatic car can not be push-started.