A planetary gearbox refers to a gear system where a number of ‘world’ gears rotate around a central ‘sun’ equipment. Typically, a world carrier is driven by the insight torque which rotates the planets around a fixed outer ring (sometimes 
referred to as the annulus). Therefore drives sunlight gear which then provides the output torque. However there are examples of the other combinations, such as the sun gear being fixed and the ring providing output torque.
The main benefit of planetary gearboxes are their high power transmission efficiencies. Typicially this can be around simply 3% per stage. Additional to the, the high effiency is delivered despite the compact design giving a large torque to size/weight ratio. A few of this is because of the load sharing occurring across the multiple planet gears.