Gears are used to change the speed and force of the motor. You can use gears to:

1. Speed up or slow down your robot.

2. Make your robot stronger or weaker.

There are many different types of gears. Spur gears are wheels with teeth.

Bevel gears mesh at right angles, so they change the direction of rotation.
Worm gears look like screws. They have many special properties.
Rack & pinion gears turn rotational motion into straight-line motion.
A slip clutch is a special type of spur gear that lets the axle continue to rotate if the gear gets stuck.
Just for fun: Look around your house and make a list of all the things that you can find that use gears.

See also: Lego gear combinations, Compound Gear Ratios, Belts & Pulleys