A planetary gearbox refers to a gear system where one or more ‘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). Therefore drives the sun gear which then provides the output torque. However there are types of the additional 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 could be around simply 3% per stage. Further to the, the high effiency is certainly delivered regardless of the compact style giving a big torque to size/weight ratio. Some of this is because of the load sharing occurring across the multiple planet gears.