What are the Cogs in the wheel well?

I noticed some gears (not really gears but a sort of sensor wheel) behind the brakes on my '94. What are they?

sounds like wheel speed sensors

OK I buy that but why are they there? I only have one Speedometer. Why do I have 4 speed sensors?

Goofy car.:lol2:

the wheel speed sensors are used to measure the speed of each wheel for the anit lock brakes and traction control systems so the control module knows which wheel is about to lock up or is slipping due to traction

Thanks again.