A planetary gearbox refers to a gear system in which one or more ‘planet’ gears rotate around a central ‘sun’ gear. Typically, a planet 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. Nevertheless there are types 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 is often around just 3% per stage. Further to this, the high effiency is definitely delivered despite the compact style giving a large torque to size/weight ratio. A few of this is because of the load sharing occurring over the multiple planet gears.