DVD region codes and PAL & NTSC colour
When the DVD format was being developed, about 10 years ago, the various film companies wanted a way to control the release of their movies worldwide. They wanted to avoid the situation where a movie might be available on DVD from abroad, at the same time as it was showing in local cinemas.
To try and achieve this they split the world in several different zones, called regions, and each region has its own code. A DVD purchased from outside the correct region won't play.
However, if you have a multi-region DVD player then you can play any disc from anywhere. Also remember that if a DVD says it is region 0 on the box, then it will play in any DVD player even if the player is not multi-region.
PAL is the colour TV system used here in the UK and across most of Europe (except France) and Australia and New Zealand.
NTSC is the colour TV system used in North America, parts of South America and Japan.
SECAM is the colour TV system used in France and most of the old communist countries (Russia, Poland, Romania etc). It is the least popular of the three systems in use today.
A useful thing to note, is that here in the UK & Europe we are region 2 and (this is important) so is Japan. All DVD players used in region 2 countries therefore have to be both PAL & NTSC compatible. This means that armed with a multi-region player here in the UK you can play DVDs from anywhere!