Thank you all!

Grab a good dictionary and find out.

Oxford Advanced Learner's is a nice choice.

Cheers.

http://www.youtube.com/watch?v=DKEM-juLxmM

http://www.youtube.com/watch?v=2nAnT3PASak

hope those links can help to answer your question.

Sheer laziness.

There are many many differences in pronunciation, vocab and even grammar (or rather, grammatical <em>tendancies</em> though these are minor...).

The biggest and most obvious pronunciation difference is the rhotic vs. non-rhotic aspect.  Where there is an "r" followed by a vowel, Americans (and Canadians) will pronounce it and British (and Australians) will not. 

Eg. For British English speakers, "sore" and "saw" are homophones but for American English speakers, they are not.

There is a lot of stuff on the internet further and better explaining this topic!  ;-)