Basic Planetary Gearset
A planetary gear set consists of three gears: the ring, sun, and pinion gears. These three gears remain in mesh at all times. The transmission is able to change the gear ratio by holding one member and driving another. This leaves the third gear to be the driven gear. There are a limited number of gear ratios a single gear set can achieve and most manufacturers apply a second planetary gear set to create what’s called a compound planetary gearset.
Ring Gear: The ring gear has internal teeth and remains in constant mesh with the pinion gears. This is the outer gear of the set
Sun Gear: The sun gear similar to our own solar system is located at the center of the planetary gear set. This gear also remains in constant mesh with the pinion gears. Do not confuse the pinion gear with the planetary gear set.
Pinion Gears: The pinion gears are located between the ring and sun gear and remain in constant mesh with both. 
This gear consists of several smaller gears contained inside a carrier called a planet carrier. This carrier contains the pins that the pinions or planet gears turn. As mentioned earlier the pinion carriers planet gears remain in mesh with the sun gear as well as the ring gear at all times. 
For instance if we hold the ring gear while driving the sun gear the pinion gear would be the driven gear. Each of these gears has a holding device so it may be held or driven according the desired gear ratio.
