Harmonic drives are a type of gearing system that use a flexible outer wheel with internal teeth, and a rigid inner wheel with external teeth, separated by a series of ball bearings. The unique design of the harmonic drive allows for high gear ratios, high torque-to-weight ratios, and precise positioning with low backlash. The compact and lightweight nature of harmonic drives make them ideal for use in robotics, aerospace, and other applications where weight and size are critical factors. However, they can be more expensive than traditional gear systems and require careful design and installation to ensure reliable performance.