To add to Sequin's answer:
NTSC was developed in America in 1954. PAL wasn't patented until 1963. It was universally recognised that PAL was by far the better system and it was thus adopted throughout most of the world.
Countries, such as the UK, had not yet started the commercial development of colour TV and so, logically, adopted the better system. The USA, however, had already spent vast amounts of money installing NTSC facilities into hundreds of studios and persuading millions of consumers to buy NTSC sets. (This was in the days when a colour TV cost nearly as much as a small car). So the USA television industry, together with its consumers, found itself stuck with the poorer system.
As has been mentioned, there's also a third system, SECAM, which is common in many parts of Africa and Asia but not used within Europe except by the French (who like to stick with their own inventions).
To complicate things further, there are variants of both the PAL and SECAM systems which are generally not compatible with other variants of the same system.
The good news, however, is that Hong Kong and the UK both use the 'I' variant of PAL:
http://kropla.com/tv.htm
Chris