Motion picture studios want to control the home release of movies in different countries because theater releases aren't simultaneous. They have required that the DVD standard include codes that can be used to prevent playback of certain discs in certain geographical regions. Each player is given a code for the region in which it's sold. The player will refuse to play discs that are not allowed in that region. This means that discs bought in one country may not play on players bought in another country.
Regional codes are entirely optional for the maker of a disc. Discs without codes will play on any player in any country. Most DVD-ROM drives let you change the region code a few times, usually between 0 and 5. Once a drive has reached the limit it can't be changed again unless the vendor or manufacturer resets the drive.
There are 6 regions.
1 Canada, U.S., U.S. Territories
2 Japan, Europe, South Africa, Middle East (including Egypt)
3 Southeast Asia, East Asia (including Hong Kong)
4 Australia, New Zealand, Pacific Islands, Central America, Mexico, South America, Caribbean
5 Former Soviet Union, Indian Subcontinent, Africa (also North Korea, Mongolia)
6 China