The old Bosch type flapper valve type is also a mass air flow sensor, just like a hot wire type. They just measure the mass differently, and the flapper type causes more restriction in intake flow. As conner said, the flapper uses a variable resistance coil and a wiper contact, like a dash light dimmer in reverse, and the hot wire type uses and electrically heated wire, and measures the current needed to maintain a given temp in the wire. The heat loss in the wire, and thus the current required, varies directly with the mass of the passing air per unit time. Note that BOTH measure air mass, not volume, as air mass is the critical factor in calculating required fuel.

AFM's are used in (Surprise!!!) mass air FI systems. The other main type is a speed-density system, which uses rpm and air density from a MAP sensor and IAT sensor instead of an AFM or MAS for air quantity in the fuel calc.

Most factory systems are mass air. Most aftermarket ecu's are speed density.