Yes, there are several regions in the world (US/Canada is one) and each has their own custom PS3. Why the different PS3s? Two reasons:
1) Different electricity standards result in different power plugs, voltages, and frequency. That difference also results in a different standard for TVs (US/Canada is NTSC, UK is PAL) which is partly due to the electrical frequency (60Hz in US/Canada, 50Hz in UK).
2) Media region controls. While PS3 games are mostly region-free, it's still up to the publisher to choose whether or not to region-lock it. So a UK game could be locked so that it only plays on UK PS3s. The same goes for movies. Fortunately, blu-ray locking (games and movies) is rare. The same can't be said for DVDs where region-locking is the norm.
I hope that helps.
PS - I should have mentioned that new LCD TVs usually can handle both NTSC and PAL signal, so that really isn't a limiting factor anymore unless you have an older TV.