Dumb Question Contest: Timing

[excelguru]

Okay guys, I'm a mechanical engineer, not a sparky as we like to call them. If it pumps liquid or blows air or holds something down, I can pretty much figure it out. But this whole "retard timing" and "vacuum advance" crap has me stumped. But I know I'm in the right place to get answers, so here goes:

I understand what advanced and retarded timing are (firing slightly before or slightly after TDC, unless I'm really clueless). I just don't understand how they physically effect the engine's performance.

And what the heck is vacuum advance for crying out loud? Does it provide more advanced timing at higher revs, based on the manifold pressure? Why?

Some people can just listen to a car idle... or accelerate... or lift throttle... and just by the sound they can tell that the timing is off and in what direction it's off. How?

Keith