The majority of DVDs are "region encoded". This is done by the studios to control the release of DVD's in certain parts of the world and often at different times by doing this in a format that is only mean't to be playable on DVD players sold/manufactured in the same country (region).
The most popular DVD Regions are Region 1 (USA) and Region 2 (UK)
A disc marked 'Region 1' (USA) is only meant to be viewable on Region 1 DVD players. Overtime the DVD player manufactures have however created more 'multiregion' DVD players which means UK DVD players can play DVD's from other parts of the world (regions). The advantages to UK uses can be:
1) US DVD's (Region 1) are often released before the UK Version (Region 2) allowing UK users to own the film/dvd in advance of the UK release.
2) Cost. Other regions can sometimes be cheaper than Region 2 DVD's.
3) Different artwork (covers) , more extras or more content. Region 1 DVD's have been known to have more extras on the disc than Regions 2. Making them more 'interesting' or 'collectable'
Below is a list of the DVD regions and DVD territories:
Region 1 - USA, Canada and US territories
Region 2 - UK, Western Europe, Japan, South Africa, Middle East, Egypt
Region 3 - Southeast Asia, East Asia, Hong Kong
Region 4 - Australia, New Zealand, Mexico, Central and South America
Region 5 - Former Soviet Union, India, Africa, North Korea
Region 6 - China
Region 7 - (reserved)
Region 8 - Special international venues (airlines, cruise ships)
1. Not all discs are region encoded. Some, but few, DVDs are meant to be viewable on any player and are known as region-free or region 0 Discs.
Video information on a DVD is usually formatted for a specific television system: NTSC (used in the United States) or PAL (Europe).
These two standards are incompatible. You need to make sure your TV and DVD player can handle NTSC. The good news is the majority of UK TV's can display a NTSC signal without any problem so owning DVD's from other regions work just fine.
Blu-ray is very much becoming the forerunner/replacement for DVD's. The good news is, at this time, the majority of all Blu-ray disc are manufactured region free!.