English pronunciation is divided into two main accent groups, the rhotic and the non-rhotic. Rhotic speakers pronounce the post-vocalic r, while non-rhotic speakers do not.
A post-vocalic r is one which occurs after a vowel such as the r in corridor, in water, or in hurt. Rhotic speakers will always pronounce an r in these words. Non-rhotic speakers may pronounce them in phrases like turn the water off where they are followed by another vowel but they will not pronounce them in phrases like put the water down where they are followed by a consonant. Non-rhotic speakers tend to lengthen the final vowel in a word like water when they do not pronounce the r. This also tends to happen in a word like hurt where the u will be lengthened and the r removed by a non-rhotic speaker.
Areas with rhotic accents include Barbados, India, Ireland, Scotland, Northumbria, southwest England, most of the northern and western United States (excluding New York and the New England region), and Canada.
Areas with non-rhotic accents include Africa, most of the Carribean, most of England (especially Received Pronunciation speakers), Australia, New Zealand, South Africa, the southern United States, New England, New York and Wales.
- http://www.alt-usage-english.org/excerpts/fxrhotic.html has some further information.