The transmission’s main job is to transmit the engine’s power to the differential while allowing different drive (and torque multiplication) ratios.
Unlike a manual transmission, where a friction clutch is used to transfer the power from the engine to the input of the transmission, the traditional automatic transmission uses a type of fluid coupling called the torque converter. It uses transmission fluid to transfer the power from the engine to the input shaft of the transmission by using two sets of nearly identical vanes. The one nearer the engine is called the turbine, and it is attached to the transmission input shaft. The one nearest the transmission is called the impeller, and turns at engine speed. If you are having problems visualizing this setup, try imagining the impeller as a pump forcing water down a tube, and the turbine as a generator (like in a hydro-electric power plant) on the other end.
At idle, the impeller is turning so slowly that not enough power is transmitted to turn the turbine and get the vehicle moving. As the engine speed increases, the turbine slowly starts spinning. The heavier the vehicle, the faster the engine needs to spin before the turbine will start moving. Eventually the impeller and turbine will be turning at approximately the same speed, however the turbine will never travel at the same speed as the impeller due to slippage. In fact, the turbine will usually travel around 3% to 8% slower than the impeller. When the accelerator is released the situation is reversed. The turbine tries to speed up the impeller, however the engine is slowed by its compression, resulting in smooth engine braking.