Fluid couplings are devices used to transmit power between two rotating shafts through a fluid medium. They consist of an impeller and a turbine, which are housed in a sealed casing filled with oil or another fluid. When the impeller spins, it creates a flow of fluid that drives the turbine, allowing for smooth acceleration and deceleration without direct mechanical contact. These couplings are commonly found in applications like automobiles, industrial machinery, and marine propulsion systems. They help reduce shock loads, improve efficiency, and provide a level of slip that can protect equipment from overload conditions.