In a pressing snatch balace, you simulaneously do a behind-the-neck press (with a snatch grip) and drop into an overhead squat. So, squat-width stance, then you press up and drop down at the same time, ending up in a full squat.
In a heaving snatch balance, it's the same thing, but you push-press the weight up instead of slowly pressing. So, squat-width stance, then you dip a bit, drive the weight up and drop into a full squat with the bar overhead.
A snatch balance is basically a more explosive heaving snatch balance. You start off with a snatch-width stance (closer than a squat stance), again with the bar behind your neck. You dip, and this time do a full jump like you're snatching, driving the bar off your back and moving your feet out to squat width, then, while the bar is moving up, you drop down into a full squat with the bar overhead.
I found a few youtube links for snatch balances:
pressing: http://www.youtube.com/watch?v=UPX1qanm ... ed&search=
regular: http://www.youtube.com/watch?v=WlabYlMk ... ed&search=
The form isn't perfect, but you'll get the idea