Gurney:
"Zones" are the publisher's way of milking the release for as much money as they can.
Just as a film may be released to cinemas in different countries at different times, and then released for TV use at different times. The timing of the release of DVDs is dealt with in the same way.
By coding them for different Zones it prevents a US DVD (Usually released first) being bought for use in UK by someone who wants to see it before the general release date.
Zones also help the distributors restrict access where the copyright status of the material may be different in different countries.