See it like this.. you boot up your computer but press nothing.
Simultaneously 20 identical computers boot up, total waist i know but effectively their all the same.
Now PC 1 is just waiting for a command, it doesn't know what the user is going to press, dosn't know the future.
But the 20 other computers could be opening all the different apps, loading them up like their all indevidualy right, but only one is.
The user opens paintbrush, now most of the 20 have opened something completely different but one has loaded paintbrush so an identical copy of that one is sent to PC 1 and it instantly loads for the user.
The same instance of the opened paintbrush can be sent to all the other machines (even if they still have opened other apps) and the process for the next user command can begin again.
It's wasteful of electricity but it can turn a dozen outdated machines into a super-computer
The tricky bit is when something done on paintbrush means that an instance of another app loaded before the command was given is outdated, entirely or in part.
Because if you look at it like PC1 is what actually happens and the others are the musings of what might be it makes a good analogy to the way the future we guess is almost always wrong.
In cluster computing virtual machines are mainly used, they exist only in-parts of all the individual machines, so instead of PC1 you use a virtual machine called Bobby.
This in where the analogy goes further because bobby is always right, makes no guess's of the future, the correct future just pop's into his head as soon as input is received
Like Bobby has achieved a state of Now mind.
But all the machines still process the possible futures, still get it wrong the majority of the time, but it doesn't really matter because they are all working together.
Their can also be as many virtual machines as their are real machines, Bobby in the now has many friends in the now also.
The funny part is that bobby an all his friends are all much quicker than the individual machines they are made of yet they are still separate units and have the same hardware. (IMG:style_emoticons/default/smile.gif
I hope i've made it clearer, you basically had it anyway.