In the simplest terms a gear is a way to generate speed or power or change the direction of power. Most gears are mounted on shafts, generally an input and output shaft. When two gears are meshing (connected) they are called a transmission.
Gears can be used to do the following things -
By connecting two gears together, Gear A with 10 teeth and Gear B with 5 teeth. Gear B will have to turn around much faster to keep up with gear A, however it does so with half as much turning force. The fact that B is turning faster than A is down to the gear ratio.
To increase the force of gears, the second gear in a pair of gears will have fewer teeth than our first gear, it will turn slower but will do so with more force.
When two gears are meshing together, the second gear always turns in the opposite direction to the first gear. So if Gear A is turning clockwise, Gear B will turn anti-clockwise. If you had three gears meshing together A and B would be turning clockwise, whilst B would be turning anti-clockwise. The middle gear in this sequence is known as an idler gear and is used when you want to keep the input and output shafts turning in the same direction.
It’s also possible to use bevel gears which can change the angle or direction of the gears by 90°. So the input and output shafts can be 90° apart.