Hi Dalel, this question is often asked here so a search (at the top) could bring up more answers. Is there something in the 'Knowledge' section about this yet? :)

DIFFERENCES BETWEEN BRITISH AND AMERICAN ENGLISH Here's a good site that has listings for both dialects: http://www.uta.fi/FAST/US1/REF/usgbdiff.html#b2a

i think the most difference is that they have different accents.

I don't know what British Tony knew, but I lived in England for 2 years and the British I knew used a good deal of slang - more than the Americans, certainly. The only British who are formal are the upper class (very upper). The language of the average Brit is rich in idioms and slang. The sight that Lenka gives is good - one thing to add: A "fortnight" (14 nights or 2 weeks) is in very common use by the British, but almost totally unknown to the Americans.

this article offers detailed explanation http://en.wikipedia.org/wiki/American_and_British_English_differences