Actually, there are two issues involved.
One, as mentioned above, is that much of the rest of the world uses different television standards than the U.S., "PAL" being the most frequent. Some DVD players will play a PAL disc and output the American standard (NTSC) to your NTSC television.
The second issue, however, is that DVDs are encoded to only play within certain regions of the world. Think of this encoding as a "key" that certain players can unlock. If you're in Region 1, theoretically, you have a "Region 1" deck that will only play Region 1... not Regions two or three for example. This was done as an anti-piracy maneuver on the part of the production studios.
The reason I say "theoretically", is that there are region-free players that will play discs from anywhere in the world.
What I can't answer is whether or not there are players that are region-free and will play back PAL video files as well.
