PFC is Power Factor Correction. Power Factor is a measurement of how effectively power is being used, and is expressed as a number between 0 and 1, with the high number meaning that power is being used effectively.
Two types of load are placed on the PSU, resistive; power converted into heat, light motion, i.e. working power. Inductive; sustaining an electric field in a transformer or motor, also called reactive power. Together, working and reactive power make up apparent power. Power factor is a measurement of the ratio between working and reactive power, or working power/reactive power. The ideal is for the two to be the same, i.e. a PF of 1.
There are two types of PFC, passive and active. Passive uses a capacitive filter on the AC input to maintain the inductive load, without an addition of power. Active uses a circuit to match the resistive and inductive loads.
Every PSU sold in the Europe is now required to have PFC, although when I bought mine, I knew nothing about this, and the dealer supplied me with a US model, complete with 110V settings. If you have a non PFC PSU, always make sure that the voltage is set correctly for your region as damage will result if not. This is another benefit of PFC; it automatically senses and adjusts to the input voltage.
The common misconception is, that PFC will lead to lower electricity bills. This is not true unless you are a commercial user who is charged by Volt Amps and PFC, rather than KW/hr.
The benefits are to the environment, keeping power clean, reducing harmonics. . For a more detailed look at PFC, see Adrian's Rojak Pot. Please note if you have a room full of PCs, e.g. a folding farm a high PFC (lowering current required) will allow you to connect more PCs, to a single circuit.