Studios created regional codes to control the different release dates of certain films.  Theater and Rental releases aren't the same.  They vary from one country to another.  For example, a movie may be released for rent in the U.S. when it is just hitting the big screen in Europe.  Another reason for the creation of Regional Codes is that studios sell the distribution rights to different foreign distributors and want to guarantee an exclusive market.  Therefore thay required that the DVD standard include different codes to prevent playback of discs in different geographical regions.  Each DVD player is given a code for the region in which it's sold.  If the disc and the players code do not match the player will not play that disc.  A disc brought in one country may not play on a player brought in another country. 

There are seven Regions.  Each assigned its own number.   DVD Players and Discs are often identified by the region number superimposed on a world globe.  If the disc will play in more than one region there will be more than one number on the globe. 

1:  U.S., Canada, U.S. Territories
2:  Japan, Europe, South Africa, and Middle East
3:  Southeast Asia and East Asia
4: Australia, New Zealand, Pacific Islands, Central America, Mexico, South America, and the Caribbean
5: Eastern Europe, Indian Subcontinent, Africa, North Korea, and Mongolia
6: China
7: Reserved
8: Specific International Venues (airplanes, cruise ships, etc. )

There is no such thing as a Region Zero 0 disc or Region Zero player.  There IS such a thing as and All Region Disc and an All Region Player.