There are several configurations for planetary systems. Typical configurations require that out of the 3 components: one is the input, one is the output, and one is stationary.

For example, one configuration is where the sun gear is the input , the annulus is the output, and the planet carrier is stationary. In this configuration, the input shaft rotates the sun gear. The planet gears rotate on their own axes, simultaneously applying a torque to the rotating planet carrier, that in turn applies torque to the output shaft (which in this case is the annulus).

The rate at which the gears rotate (gear ratio) is determined by the number of teeth in each gear. The torque (power output) is determined by both the number of teeth and by which component in the planetary system is stationary.
Planetary Gear Reducer Advantages
The Planetary Gear Reducer arrangement is an ingenious engineering design that offers many advantages over traditional gearbox arrangements. One advantage is its unique combination of both compactness and outstanding power transmission efficiencies. Typical efficiency losses in a Planetary Gear Reducer arrangement are only 3% per stage. This type of efficiency ensures that a high proportion of the energy being input into the gearbox is multiplied and transmitted into torque, rather than being wasted on mechanical losses inside the gearbox.

Another advantage of the Planetary Gear Reducer arrangement is load distribution. Because the load being transmitted is shared between multiple planets, torque capability is greatly increased. The more planets in the system, the greater load ability and the higher the torque density.

The Planetary Gear Reducer arrangement also creates greater stability (it"s a balanced system) and increased rotational stiffness. Disadvantages to the planetary arrangement include design complexity and accessibility issues.

In the drawing above, the Fixed Axis Gear System is the traditional arrangement where a pinion is driving one large gear on a parallel shaft. In the Planetary Gear System arrangement, one or more gears (planet gears) surround the pinion (sun gear).
The two systems above have approximately the same ration and same volume. However, the planetary system has 3 times higher torque density and 3 times higher stiffness due to the increased number of gear contacts.
Fixed Axis Gear System:
　　•Volume = 1, Torque = 1, Stiffness = 1
Planetary Gear System:
　　•Volume =1, Torque = 3, Stiffness = 3
Other gearbox arrangements (as mentioned previously) include bevel, cycloid, helical, harmonic, spur, and worm.