I've noticed that some amps instead of just having gain have pre and post gain. I know that gain affect the distortion of the amp, but I don't know the difference between pre and post gain. What is the difference between pre and post gain and is there a reason it is split up?
Most guitar amps have two amplification stages:
The pre-amp which takes the very low signal levels output by the guitar and amplifies them to a higher level, approximately like line-level.
The power-amp which takes the line-level signal and further amplifies it to drive the speaker.
These are separate since different design considerations are important in amplifying the original signal on one hand, and sending a hefty amount of power to the voice coils in the speaker on the other.
The two amplification stages have different distortion characteristics when they are over driven.
It's complicated, but the simple answer is:
Post makes it louder. Presuming you have plenty of headroom in your post amplification stage, that should be all it does.
Pre drives the input harder, causing more distortion. It does not necessarily make the overall output any louder, but it will tend to compress the peaks in a fuzzy sort of way, and if you keep turning up the pregain you'll simply hear a big mess.