Help understanding USB chargers. Are they wattage unique?

I need some help to understand how USB chargers work.
Ive got friends and family asking me (and actually I am very curious myself) if USB chargers are interchangeable. Now, I don't mean the wire, because obviously micro male to micro female and mini male to mini female, etc. But the base that actually plugs into the wall outlet or power strip, how can I know which can be used on which? Im under the impression that a Motorola Charger base (just the square part the plugs into the outlet) cannot be used to charge Samsung galaxy tabs/notes/S' as there is some sort of special power signal or governor programs for their respective products. Another reason I would especially like to learn about this is that I myself would love to be able to install a USB wall outlet. like this
http://imwm.org/useful-usb-socket-design/ or any USB wall outlet for that matter.
So I would need to know what power wattage need to go to each USB female port. Like I pointed out earlier, Im under the impression that not all USB put out wattage/signal equally. So if there is a 4 USB port wall outlet, would all of them be able to charge Samsung, Motorola, and Apple products interchangeably, or would each have to have a specific wattage.