There are six world Regions for DVD.
Discs manufactured in one region will usually only play on players that were manufactured in the same region, so discs bought in the USA will not play on U.K. players and vice versa. You can purchase multi-region or region-free DVD players, but do LOTS of research on these before purchasing. Some have been converted manually, which voids the warranty with the manufacturer.
However, the regional coding system is entirely optional and discs without Regional Codes will play on any player in any country.
Canada, U.S., U.S. Territories.
Albania, Andorra, Austria, Bahrain, Belarus, Belgium, Bosnia and Herzegovina, Bulgaria, Canary Islands, Croatia, Cyprus, Czech Republic, Denmark, Egypt, Estonia, European Union, Faroe Islands, Finland, France, France, Georgia, Germany, Gibraltar, Greece, Greenland, Hungary, Iceland, Iran (Islamic Republic of) Iraq, Ireland, Israel, Italy, Japan, Jordan, Kuwait, Latvia, Lebanon, Liechtenstein, Lithuania, Luxembourg, Macedonia (the Former Yugoslav Republic), Malta, Moldova, Principality of Monaco, Netherlands, Norway, Oman, Poland, Portugal, Qatar, Romania, Russian Federation, San Marino, Saudi Arabia, Slovakia, Slovenia,South Africa, Spain, Sweden, Switzerland, Syrian Arab Republic, Turkey, Ukraine, United Arab Emirates, United Kingdom, United Kingdom (Channel Islands) Vatican City State, Yemen, Yugoslavia
Southeast Asia, East Asia (including Hong Kong)
Australia, New Zealand, Pacific Islands, Central America, Mexico, South America, Caribbean
Former Soviet Union, Indian Subcontinent, Africa (also North Korea, Mongolia)
Special international venues (airplanes, cruise ships, etc.)