What is the difference between the American and British accents?

Originally, both the British and Americans spoke with a rhotic accent. Rhotic essentially means an accent where the letter 'r' is pronounced strongly after a vowel. Rhotic accents are strong in both the US and Scotland, but seem to have disappeared from British English and its derivatives, such as Australian and New Zealand English.

The dominant American accent (the typical accent of the mid-western US) is rhotic, and British English is, as a rule, non-rhotic. Typically, US speakers pronounce every "r", wherever they appear in a word. Most British speakers (and you will note that there are some British Isle accents that are strongly rhotic, like US) do not pronounce every "r". Americans emphasise the "r" at the end of words such as "teacher" or "neighbour", but in many non-rhotic British accents it is more of a "schwa" (an unstressed, neutral, toneless vowel sound) so it comes out as "teacha" (unstressed) or "neighba".

This main difference also varies across the country of origin. For example, the US has a clear distinction in the accents between inhabitants of the north and south, not to mention less clearly defined differences across the states. Similarly, British people have a different accent according to their locality. The "cockney" accent is vastly different to the middle and upper class accents.

Another common difference appears to be that Americans pronounce words such as 'herbal' without the 'h'. Syllables may be stressed differently, too. "Oregano" in the US tends to be pronounced "o - reg - a - no" whereas in British English it is more likely to be "o - reg - AH - no".