Power factor is the ratio of actual AC input value when compared to power consumed by the various PSU system circuitry. Power supplies without PFC usually only have an actual PF rating of 50% (the rest is turned into heat energy). With PFC, first and foremost you’ll notice that your system has better voltages as the on-board circuit controls the individual lines. An active PFC power supply will also draw less power, and will cost you less in electric bills over time.