The World is divided into several DVD regions. DVD players can only play DVDs that are coded for a specific region. In other words, you cannot buy a DVD in Japan (which is in DVD Region 2) and play it on a DVD player in the U.S., which is in DVD Region 1.
However, there are DVD players available that can bypass the Region Code system, which some exceptions. This type of DVD player is referred to as a Code Free DVD player.
For a full explanation on DVD Region Codes and Resources for Code Free DVD Players, refer to my article: Region Codes - DVD's Dirty Secret.

