PAL and NTSC are standards for different countries. They are incompatible. Basically, PAL has 625 lines of resolution and NTSC has 525.

As Geoff said, there are players in the UK that will play both, but they are not as readily available in the U.S. There are issues with conversions that contribute to that, and the few mult-standard decks that are availabe in the U.S. are very expensive.

I'm honestly not sure about computers. Your program that plays DVD's in the computer might be able to give you the answer.