A planetary gearbox identifies a gear system in which one or more ‘world’ gears rotate around a central ‘sun’ gear. Typically, a world carrier is powered by the input torque which rotates the planets around a set outer ring (sometimes known as the annulus). This in turn drives the sun gear which then supplies the output torque. However there are examples of the various other combinations, such as the sun gear being fixed and the band providing output torque.
The main advantage of planetary gearboxes are their high power transmission efficiencies. Typicially this is often around just 3% per stage. Further to the, the high effiency is definitely delivered regardless of the compact 
style giving a sizable torque to size/weight ratio. A few of this is thanks to the load sharing occurring across the multiple planet gears.