Stupid question - please explain 'dual system' to me...

Question

I'm looking at a 2nd hand mac. Can someone explain to me what dual systems are and how they work? Why do they make dual systems? What is the advantage This is what I'm looking at:
APPLE MAC G4 QUICKSILVER TOWER DUAL PROCESSOR RUNNING 10.4.11 COMPLETE SYSTEM
THIS MAC HAS TWO 867MHZ(1.734HZ)PROCESSORS WORKING AS ONE. I'm really looking to expand my current G4 but as this 2nd hand one has a whole bunch of software I'd love to get my hand on it seems quite good value.
I know it's a very basic question but I've done a search and cannot find an explanation, I'm not afraid of appearing dumb haha!

Answer 1

"Dual system" usually means having two operating systems on your computer so your computer can boot to either kind. For example, I have both OSX Tiger and OS9 on this computer. I can boot to either.
What you indicate is a dual processor computer. It has two processors inside. Some programs (especially video and graphics) require a lot of processing power. If you do video on a single processor computer it can tie up the computer. If you want to do other work, other than maybe the simplest task, you may find the computer slows down a lot because the video is basically hogging the single processor. If you have a dual processor computer it can handle multiple processor-hungry tasks at one time and you can use the computer normally. Note that having two processors often doesn't do a task any faster than a single, except to the extent that the computer isn't waiting for other things to finish. A video will process twice as fast on a 1600 MHz (1.6GHz) single processor computer than dual (2x) 800 MHz. However, if you want to do a lot of other work while it is processing then the 1.6GHz computer will be tied up for two hours and you won't be able to do much else during that time. Although the task may take 4 hours on a slower, dual processor computer, you can continue working while it is doing the processing instead of sitting around waiting. In that regard dual processor computers can be "faster" (at least in terms of your productivity) than faster single processor computers.
So, if you do a lot of CPU intensive work (graphics, video) a dual processor can be worth it, and sometimes even better than a faster single proccessor, but they cost more. If you don't do that kind of work then you can usually get a faster single processor computer for the same money and that will be a faster computer for how you use it.
Another way to think of it is in terms of single and multi-lane highways. During rush hour it may be faster to go on a single-lane highway with a slower speed limit than a crowded multi-lane with a higher speed limit where if the traffic is so heavy a single lane gets crowded an maybe slower than the multi-lane.