These computers are single purpose, hidden, small, and low cost. They probably don't have a screen or keyboard but they are still computers. These hidden computers, that serve machinery, rather than people directly, are called embedded computers. The devices they control are called embedded systems. Real-time computer systems are a subset of the computer world where timing is important.
Timing is important in areas like robotics and machine control.
An example of a real-time computer system is the ignition system in most contemporary cars. If the car computer doesn't fire a spark plug on time, the car runs rough or loses power. Another example, is the timed recording capability of most VCR's. From the moment you finish programming your VCR to tape a show in your absence, to the time the show begins, the computer in your VCR is just waiting. The computer doesn't do anything until the show begins, guaranteeing the taping starts on time.