Couplings are mechanical devices that connect shafts to transfer power, absorb shock, accommodate misalignment, prevent heat transfer, and provide overload protection. Types include rigid, Oldham, gear, flange, universal, split muff, diaphragm, flexible, fluid, constant speed, bush pin, sleeve, jaw, and bellow couplings, each with unique applications. Materials used are steel, cast iron, aluminum, brass, and bronze. Key considerations in coupling design include misalignment, torsional stiffness, torque, axial compliance, backlash, and torque uniformity. Choosing the appropriate coupling ensures efficient, reliable performance in mechanical systems.