A gear train is a connected set of rotating gears by which force is transmitted or motion or torque is changed. The gears connect through interlocking teeth. Meshed gears always turn in opposite directions.

A compound gear is two gears that appear fixed together. (B &C)

An idler is the middle gear between two gears that allows the outside gears to rotate in the same direction. (B & C)

The driver is the gear where the force is applied.(A)

The driven or the follower gear is the gear that is moved due to the driver.(D)

linear: straight in one direction

oscillating: back and forth in an arc

rotary: around in a circle

reciprocating: back and forth in a straight line