A mechanical gyroscope uses the resistance to torque of a spinning object to measure movement. The toy referenced by that other guy isn't even a measuring device, it merely demonstrates the principle. An navigation grade mechanical gyroscope is very precisely machined, uses exceptionally low friction bearings, is often housed in a vacuum chamber, is driven by very quiet motors, and uses very sensitive electromechanical sensing devices to measure its resistance to motion. It costs tens of thousands of dollars, not the $3 and change the other guy mentioned.
A LASER ring gyro measures the interference pattern between two LASER beams being guided by circular optical fibers in opposite directions. Rotational motion induces a phase shift between the two beams. Depending on size and required precision the cost can be tens of dollars to tens of thousands of dollars. It uses very precise electro-optical sensors to measure that interference. Last time I checked (about 15 years ago), the SDN500 Inertial Navigation System cost in the neighborhood of $30,000.