Master volume - essentially this determines how much power amp distortion/coloration you are getting.  as you turn it up, you'll get compression, then distortion, depending on the amp model.  This parameter also affects the other parameters.  At 0%, basically none of the other parameters do anything.

Sag - turned up this can make a distorted sound be a bit more saturated sounding; it's like you get the bass response before the high-end crispness.  but if you set it really high there is actually a barely noticeable delay between your playing and when you hear the brunt of the notes through the amp model.  turned down all the way seems to give a very tight response, but it can sound a little too bright and crisp.

Bias - turned up = more headroom; turned down = less headroom.  But it will also change the nature of the distortion when the headroom is reached.

Bias X - this basically refers to the dynamic range of the headroom.  So if you pick hard you break the headroom and get distortion but if you pick softly you don't... or it's always maxing out the headroom or always isn't.  i dont' remember which way does which, sorry.  i tend not to adjust this parameter.

Hum - I can't really tell you.  This parameter seems quite subtle to me, but it should change the nature of the power amp distortion.