Answer:
A pipe union is a three piece fitting with a big nut in the center. Loosen the big nut and the joined pipes will separate.
Are you sure you don't mean a coupling? A coupling is a one piece tube with female threads all the way through it. These are trickier to untighten because by loosening one side of the fitting, you'll be tightening the other side, which will usually bottom out the fitting on the side that's being tightened before the side that's being loosened comes out of the fitting. In short, the way to untighten a coupling is by either cutting the pipe, spinning the coupling and replacing things as needed OR by spinning the entire run of pipe...