The Channel Volume controls the level of the preamp for that channel. The Master Volume controls the drive level into the power amp, which affects the total loudness. With the Channel on 1, and the Master on 10, there would be very little music coming out of your speakers, but since the power amp would on full, there might be quite a bit of 'hiss'. With the Channel on 10, and the Master on 1, that channel of the preamp would be up full, (and probably distorting), but the power amp would be sending very little power to the speakers, so you would hear a quiet, but distorted sound.

'Pre' and 'Post' refer to the position of the tone controls relative to the gain stage. 'Pre' controls the tone settings before the signal gets boosted; 'Post' works on the sgnal after it gets boosted.

'Gain' is the amount of boost or amplification that is applied to the signal. Unless you have active pickups, with a preamp built into the guitar, all the guitar controls can do is reduce the amount of signal that comes out of the guitar. From there, gain is applied at any point in the signal path that you want it.