Here is a definition that Arcs posted a while back....
"PID stands for Proportional, Integral, Derivative. It is a type of control scheme that does a bunch of math based upon the difference between what the temperature is, and where it should be. PID control schemes reach the setpoint fastest, with the least overshoot, and least error.
Having said all that, other control schemes work quite well. Your house thermostat is an on/off control that is very accurate (it helps that, to change the house temperature, it takes some time; there is a lot of thermal inertia)."
Needless to say... The inner workings of the PID is "over my head" but I do have one and it works good.
Mike