it mostly has to do with the countries being far apart for most of the last 400 years, so the language developed a little bit differently. As QM's link suggests, some of it was deliberate. Words are still being invented and changing (eg to do with computers) but modern communications mean the two countries are more likely these days to use the same ones, with the same spelling. The Americans couldn't care less how Brits spell things, though the British seem more sensitive about Americans changing 'their' language.