Aren't computers the machines sitting the computer lab? Yes, those are definitely computers. However, there are many more types of computers. For instance, there is a computer inside every cell phone. Most cars have multiple computers performing various functions such as anti-lock braking, engine management and environmental control. When a computer is not visible (but performing useful functions), it is called an embedded computer.
There are also huge computers that span entire rooms. There are often called super computers. They are specialized to perform complex mathematical operations very quickly. Some are used to simulate how the structure of a car would crumble in a collision, others are used to simulate how a bomb would explode.