DVD region codes prevent people from around the world from watching each others DVDs. There are a total of six different DVD region codes, listed below. They were set in place at the birth of DVD by the US Motion Picture Association and other organizations in order to keep people from buying and playing DVDs from other regions. This is the companies' way of controlling their markets and keeping them separate. For example, many American movies are released later in Europe and Japan, as well as elsewhere, than they are released in the US. If American DVDs worked in European players then there would be less stopping people in Europe from buying the US version early, before it is released in their home country. This bypasses their local market, and the movie companies don't want that.

Most DVD Players are supplied with the region code set to the country in which they are sold and her in the UK this is Region 2. Some players are advertised as multi-region (that is will play any disc from any region)

Technically there is no such thing as a region zero disc or a region zero player. There is such thing as an all-region disc. There are also all-region players. Some players can be "hacked" using special command sequences from the remote control to switch regions or play all regions. Some players can be physically modified ("chipped") to play discs regardless of the regional codes on the disc. This usually voids the warranty, but is not illegal in most countries. Many retailers, especially outside North America, sell players that have already been modified for multiple regions, or in some cases they simply provide instructions on how to access the "secret" region change features already built into the player. The most popular region code hacks can be found on videohelp.com

PAL / NTSC
There are two formats: PAL (Phase Alternate Lines) and NTSC (National Television Standards Committee).
NTSC is used mainly in North America, Japan, Mexico and Canada, and PAL is used almost everywhere else in the world including Australia and NZ.

Most televisions and DVD players have been designed over the past few years to support both PAL and NTSC. But say for example if you play a NTSC DVD on a PAL ONLY system, and vice versa, the picture on your monitor will be distorted.

PAL vs. NTSC
The U.K. PAL format is technically superior to the US NTSC television format. DVDs released in the U.K. have 25% more lines of resolution area than their USA counterparts. Moreover, the color quality of PAL is superior to the US NTSC format. The local DVD product therefore has a potential quality advantage over the foreign product. I say potential because some PAL DVDs have been very poorly authored. While this is a reason for someone to use the region 2 version of a movie, furthermore there can be differences between the content. By contrast, PAL versions of movies run 4% faster than their NTSC counterparts (or the theatrical release), and the resultant speed change comes through as a pitch shift. What this means is that music and audio play just 4% faster, which changes every note of music and makes people speak with a slightly higher pitched voice. In addition, the movie length of the PAL release is reduced by 4% from the theatrical release. This does not worry most people.

Depending on your equipment there are several ways round this and I will basically discuss the UK side of this although the USA is similar but in reverse. Most (but not all) of the modern DVD Players have the ability to convert an NTSC disc to output PAL 50hz and this is viewable on all domestic TV receivers via the Phono RCA Jack, S Video, or Scart inputs as Composite Video or S Video. A few DVD Players output as PAL 60hz which can be viewed satisfactory on the modern TVs but will result in a rolling picture on older TVs. If your DVD Player will not output PAL the resultant picture on an older TV will be in black & white and rolling. Most DVD Players have a Scart & or Component Video output which outputs the colors & synchronizing signals separately and this can be connected directly to the Scart or Component video input of a modern TV. This gives the best quality picture and the signal is independent of any PAL/NTSC format.