There are two basic types of systems, direct and indirect. You have an indirect system.
Direct systems use a pressure sensor and transmitter for each wheel. They send pressure information from each wheel to a computer module on the car. Some of these systems provide real time pressure and temperature readings to the driver.
Here is an example of one of the sensors
Indirect Systems look at the wheel speed of each wheel (Using the ABS wheel speed sensors) and compares its speed to the others. If a tire is under-inflated it will spin faster because the tire is smaller. These systems don't know which tire is flat, just that a tire is flat, or maybe not flat just less than the other tires. There are some advance versions of this system that use signal processing to detect all of the tires being under inflated. Since they would all rotate at the same speed if they all had the same amount of under-inflation.