There are two general types of tire pressure systems,
direct and
indirect. An indirect system doesn't actually measure the tire pressure. It uses the wheel speed sensor (part of the ABS) to measure very minute differences in rotational speed between all four tires. When it senses a difference from what a nominally good tire is, it will turn on the tire pressure light. Note that you will need to check the tire pressure in all tires, as it doesn't indicate which tire is the problem. Once the tire pressure is adjusted, reset the light according to the owner's manual. You'll also need to do this after changing to a spare.
Direct systems attach a pressure sensor/transmitter to the vehicle's wheel inside the tire's air chamber. This system uses a small battery-powered pressure sensor with radio transmitter on the back of each tire valve, attached to the wheel rim. Some direct systems use a sensor mounted to a metal band around the inside of the rim. The batteries are designed to last 10 years. There will be a corresponding radio receiver next to each wheel, which picks up the signal from the sensor and passes it to the system's in-car receiver and the information is displayed to the driver.