There are three basic technologies used for IMU's: MEMS, FOG, and RLG.
MEMS, Micro Electro-Mechanical Systems, uses silicon chips with integrated tuning forks as sensors. These are the most popular sensor of the three, are the least expensive, and also the least accurate. They are typically small, rugged, and consume little power. They are generally inexpensive to produce, have a great future potential, are reliable and have a low maintenance schedule, and have a tremendous potential for miniaturized solutions (UAS).
FOG, fiber optic gyros, consist of a coil of optical fiber using the interference of light to measure angular velocity. They are better than MEMS and have reached and even surpassed RLG sensors performance. They have a very long life relative to other technologies. They are typically more expensive than MEMS and not as expensive as RLG sensors.
RLG, ring laser gyros, were generally considered to be the most accurate, but this changes over time of course. They are generally quite expensive.
RLG and FOG sensors are both based upon the same basic theory. The essential difference is that the RLG laser beams do not use fiber optics but instead use mirrors to direct the laser beams. In both sensors, the larger the better for accuracy as the path of light is increased. Both start up very fast as they are solid state devices (no moving parts).
Early on RLG units ruled the day. Then FOG sensors became more dominant. As of 2016, MEMS sensors by far outnumber all others. Performance has gone up (but it doesn't reach the really high-end FOG or RLG units) while size has drastically decreased along with costs.