The guide numbe is an indication of the power but can also be used to work out settings manually,
Guide numbers are usually given for ISO 100 and in metres now (although they used to be in feet).
If you divide the guide number by the distance, you gert the aperture setting you need to use at ISO 100.
So if a flash has a guide number of 60m at ISO 100, at a distance of 4 metres the aperture you need is 60/4 = 15 so you would use f16 as it is close enough.
Today, not many people use flashes manually so it's just used as a relative measure of power. The higher the number, the more powerful it is.
If you have a specific purpose for a flash and you know the maximum distance you will be using it, and the likely highest aperture, you can work out what the minimum requirement would be in terms of guide number (distance x aperture) and discount any products with a lower number.
Equally, if you are looking at a specific flash and know its guide number, you can work out the maximum distance it will work at various apertures.
EDIT:
Sorry mods, I've just found out on the internet at a website the answer to my question, thanks guys
Perhaps I should have read that bit too!
Steve.
