With automatic transmissions, torque converters usually take on the job of the clutch and transmit the forces between the engine and gearbox.
As a rule torque converters are operated hydrodynamically, ie: they transmit the drive torque of the engine by means of the flow forces of a liquid, usually oil.
The torque converter is effective especially when moving off (engine rotates, car stands still) by its ability to steplessly transmit torque and rotational speed.