MAP - Manifold Absolute Pressure. Measures, as the name suggests, the pressure present in the inlet manifold and, together with engine speed, manifold air temperature and a value in the ECM for volumetric efficiency at that engine speed, calculates the mass of air drawn into the engine. The delivered fuel desired is based on that. The 'absolute' refers to the fact that it is referenced against a vacuum rather than atmospheric pressure.
MAF - Mass Air Flow (meter) directly measures the mass of air flowing into the engine. In Commodores this is done by measuring the amount of electrical energy required to maintain a (heated) wire in the air flow at a constant temperature.
In terms of power potential produced by the engine there is no difference between each type of system. Each does have advantages. MAF systems, by directly measuring airflow, have a greater flexibility in adapting to modifications that alter volumetric efficiency but may represent a (small) restriction to airflow.
MAP, also known as '(engine) speed - (air) density', systems don't have an airflow meter in the intake path but, relying on a 'map' (as in plot) of volumetric efficiency to calculate the air flow into the engine, are less able to adapt fuel flow to large alterations in volumetric efficiency.