A planetary gearbox refers to a gear system in which a number of ‘planet’ gears rotate around a central ‘sun’ gear. Typically, a world carrier is powered by the insight torque which rotates the planets around a fixed outer ring (sometimes known as the annulus). This in turn drives the sun gear which then supplies the output torque. However there are examples of the various other combinations, such as the sun gear being set 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. Further to this, the high effiency can be delivered regardless of 
the compact design giving a huge torque to size/weight ratio. A few of this is because of the load sharing that occurs across the multiple planet gears.