A planetary gearbox refers to a gear system where a number of ‘planet’ gears rotate around a central ‘sun’ gear. Typically, a world carrier is driven 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 types of the various other combinations, such as the sun gear being set and the ring providing output torque.
The main benefit of planetary gearboxes are their high power transmission efficiencies. Typicially this can be around just 3% per stage. Additional to this, the high effiency is usually delivered despite the compact style giving a huge torque to size/weight ratio. Some of this is because of the load sharing 
occurring across the multiple planet gears.