A planetary gearbox identifies a gear system where one or more ‘world’ gears rotate around a central ‘sun’ equipment. Typically, a planet 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 provides the output torque. However there are examples of the other combinations, like 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. Additional to this, the high effiency can be delivered regardless of the compact style giving a large torque to size/weight ratio. A few of this is because of the load sharing occurring across the multiple planet gears.