DVDs have "regions", not "zones". There are a LOT more than just 2...at least 7, I believe, covering various parts of the Earth.
The reason these regions were created was to satisfy the movie industry.
Sometimes a movie may still be in theaters in one part of the world, when it's already being released on home video in another.
The movie industry was afraid if, for example, a movie came out on DVD in the United States, people in other countries would somehow get copies of the DVD, and not bother to go to the theater to see the movie. They'd just watch it at home.
Create "region" differences, and a U.S. disc can't play on a U.K. DVD player (or anywhere else outside the U.S. or Canada).
NTSC and PAL (along with SECAM) are different TELEVISION formats, that mostly have to do with how the picture is converted to electronic impulses and then changed back into a picture again.
This means that if you bring a U.S. disc to the U.K., you're fighting both the TV format differences, AND the region coding.
Oddly enough, many DVD players will play both NTSC and PAL, and just put out the picture in whatever format you tell it you need. (Set it for "NTSC TV", and it'll put the picture out as NTSC. Tell it "PAL", and that's the type of signal your television will be fed.)