Aux sends may be used for two different purposes: to send signals to an external processor or to send signals to a separate monitor mix for the musicians onstage. Pre-fader aux sends are generally used for the monitor mix, because the channel faders don't affect the signals sent from the aux sends. Any adjustments made to the front-of-the-house mix don't affect the balance heard by the performers onstage.
Post-fader aux sends are generally used for connecting external effects or dynamics processors--the signal is sent to the processor from an aux send and reenters the mixer through an aux return. Each channel has its own aux send control, so you can adjust the amount of processing. The effected signal will be raised or lowered by the channel fader.
Musician's Friend carries a variety of mixers that are well suited for live performance applications, by Mackie, Yamaha, and Alesis, among others. On most mixers, there is at least one pre-fader and one post-fader aux send, and on some mixers there are aux sends which are switchable between pre- and post-fader.