Pre Fader and Post Fader are both related to the Efx/Aux Loop. To put it as simply as I can, if a signal is sent into the Efx/Aux Loop Post Fader then it's volume will be affected by changes in the level of the Channel and Buss Faders. Thus, as you increase the volume on a Channel or Buss you also increase the level of the signal going into the Loop. And, likewise, if you decrease the volume you decrease the level going into the Loop. This is the normal mode when adding Effects to a mix as you want the volume of the Effects to increase or decrease as you move faders up and down.
If the signal is sent to the Loop Pre Fader then the volume remains at whatever level you set it at, regardless of what you do with the Channel or Buss Faders. This mode is normally used either for Providing a Monitor Mix for the Talent that is independent of the Main Mix (so you can make adjustments to your heart's content without affecting what they're hearing) or Foldback for musicians onstage (which is essentially the same idea). You can use a Pre Fade Send to drive an Effects Unit, but it's more in the nature of a novelty than anything serious (like fading out a reverbed vocal to leave only the reverb).
Clear as a swamp?