This document provides an overview of gears, including their classification, types, terminology, and applications. It discusses the main types of gears such as spur gears, helical gears, bevel gears, worm gears, and rack and pinion gears. It also describes gear trains such as simple, compound, reverted, and planetary gear trains. Applications of gears include transmitting motion between parallel and non-parallel shafts, as well as converting rotational motion to linear motion. Gears are useful for transmitting power in machines while allowing for changes in speed and torque.