A planetary gearbox identifies a gear system where a number of ‘world’ gears rotate around a central ‘sun’ equipment. Typically, a planet carrier is driven by the input torque which rotates the planets around a fixed outer ring (sometimes referred to as the annulus). This in turn drives sunlight gear which then supplies the output torque. Nevertheless there are types of the other combinations, such as the sun gear being fixed and the band providing output torque.
The main benefit of planetary gearboxes are their 
high power transmission efficiencies. Typicially this is often around simply 3% per stage. Additional to the, the high effiency is certainly delivered regardless of the compact design giving a sizable torque to size/weight ratio. Some of this is thanks to the load sharing occurring over the multiple planet gears.