Article taken from club-s12.org forums. Posted by Magnum

MAP Sensor
Manifold absolute pressure sensor. Unrelated to the MAS. This sensor is mounted inside the intake manifold and provides a signal to the ECU of the exact intake manifold pressure. MAPs are sometimes used with electronic boost controllers to sense the amount of boost that the engine is running.

Mass Air Sensor
The MAS (also called the air-mass sensor, or the mass air flow (MAF) sensor) detects the volume of air entering the engine and tells the ECU, which uses this information to decide how much fuel to supply to the engine.