Epicyclic gears are the gear system used in a conventional automatic transmission.
Epicyclics, also known as planetary gears, have a central (sun) gear which meshes with two or three small planet gears, which are connected by a U-shaped planet carrier. The planet gears also mesh with an annulus, a ring-shaped gear with teeth on the inside.
In an automatic gearbox there are several epicyclic gear trains. The sun gears act as the input into each epicyclic train, and the planet carrier is the output. The annulus of each epicyclic system can be braked to prevent it from moving, and this changes the output speed of the planet carrier.