When DVDs were introduced, they were made with the ability to encode the digital data. Hollywood insisted that different areas of the world have different regions so that they could regulate when films were introduced.
Traditionally, North America gets the release several months before Europe and Australasia. The film viewed in the European cinema has already been viewed in the USA. It is washed and then flown over to Europe for distribution in our cinemas - keeps the costs down. It would be no good if the film was on DVD in America and at the Cinemas in Europe. People would buy the film from America rather than watch it in the cinema and the film companies would lose money. So the world was carved up into regions:-
0 No Region Coding
1 United States of America, Canada
2 Europe, including France, Greece, Turkey, Egypt, Arabia, Japan and South Africa
3 Korea, Thailand, Vietnam, Borneo and Indonesia
4 Australia and New Zealand, Mexico, the Carribean, and South America
5 India, Africa, Russia and former USSR countries
6 Peoples Republic of China
7 Unused
8 Airlines/Cruise Ships
If you buy a DVD player, it is supposed to only play DVDs available in your zone. However, you can buy "chipped" players capable of playing all zones. I've got a chipped player so can watch DVDs from any county in the world

Great for a film buff like me.
Thankfully, they can't do that with books (although they may try with electonic books - who knows?). Titles are often released eariler in the US than in Europe, but with the net, you can order stuff from anywhere really so it becomes less of a problem.
Marie, I've also noticed that there are different covers for books depending on the country. The US often has very different covers to the European releases. I may add some of this info to our new library reviews system which will be available soon.
And why was Harry Potter's title changed to "Harry Potter and the Sorcerer's Stone" anyway?