With a MAF the computer will base air/fuel calculations on air entering the motor instead of a set program (Programmed to stock motor) adjusting fuel only on manifold pressure.
By doing this your computer can immediately see more air coming into the motor and adjust fuel to meet the requirements of the new air coming in. If you go wild enough you will need to get some kind of tuning either way because there is only so much a MAF can compensate for in terms up upgrades.
In short its probably the best upgrade you can do to your car if you plan on changing cam, heads or intake.
Chris