There are two types of TPMS direct and indirect.
This uses a sensor inside the wheel/tyre which sends a signal to the TPMS ECU; this can then either display the pressure for each tyre or have a single warning light to illuminate when the pressure is below normal. These sensors are normally fitted with a battery with a 7-10 year life.
These use the abs sensors to monitor individual wheel speeds, an under-inflated tyre will have a slightly smaller overall diameter compared to a correctly inflated tyre, this tyre therefore has to rotate faster compared to the other tyres, the ABS sensor sense this speed increase and warns the driver of a flat tyre. This system will have a calibration button which must be pressed when the tyre pressures have been correctly set.