You don't ask much bumber
In a nutshell:
Water Fed Pole window cleaning uses pure water to clean the windows.
The water you are using has been through a purification process that has removed al of the impurities and disolved solids.
Once you have washed the windows, and rinsed them, only the purified water is left on the glass and frames.
When the water evaporates glass and frames are left spotlessly clean as there is nothing in the water to be left behind and stain the glass.
Were you to use tap water, once the water evaporates, the mineral solids that were disolved in the water would then be deposited on the glass.
Should you buy one of the WFP systems, it would come supplied with a water purification system.
You would attach this to your household supply, and though systems vary, from the purified water being fed into a holding tank near your mains supply, to being fed directly into whatever you would be using to work from, ie, a large tank in a van for instance, all of the systems do the same thing.
For someone with a van mounted system, they would have their water pumped from their van using a large leisure battery, and a demand feed pump, through hose to the brushhead.
Check out the many postings on this site for much more detailed answers to any other queries you may have.
If you know little about WFP, you will learn all there is to know on this and other forums.
It is generally quicker than traditional methods, a darn sight safer to, as the use of ladders is almost eliminated completely.
It is possible to do work up to 80ft off the ground.
Ladders are generally only used for access.
A great many WFP users have changed over to this method of window cleaning after many years working traditiionally as you now do.
You will be hard pressed to find any that will not say it is the best thing they have ever done.
It's a big subject, and can't really be covered in a single reply.......not without boring the pants of everyone reading it
Ian