As the topic denotes its all about transmission system.A fluid coupling or a torque converter is usually used in place of clutch in a vehicle especially for automatic power transmission system.
fluid coupling consists of two(turbine and impellor) rotors which form the basic structure and its very helpful in trabsmitting high speed to tge wheels.
Torque converter on the other side has similar construction and a extra part called stator between two rotors.It helps in guiding the fluid and decrease the loss.
Torque converter is mainly used to produce high torque at low speed.
In both of them working medium is fluid and if we compare the usage and advantage torque converter has a edge over fluid coupling because it can act as torque producer at low speeds and at high speeds it act as fluid coupling to produce high power trabsmission at high speed.