In addition to the spelling differences Abood listed, there's also some words which have a double "L" in British English and a single "L" in American English. For example: cancel -> cancelled, cancelling (UK) cancel -> canceled, canceling (US) travel -> travelled, travelling, traveller (UK) travel -> traveled, traveling, traveler (US) There is also -ise vs. -ize. In the US, we realize, criticize, and theorize (but we still exercise); in the UK, people realise, criticise, and theorise. A fun fact that you may not know is that most of the spelling differences were introduced by a man named Noah Webster, who wrote the first major American English dictionary. He didn't like the standard spellings of certain words, so he just went ahead and changed them to the versions he liked, and that became standard in the US!

Grammar Americans use the present perfect tense less than speakers of British English and a British teacher might mark wrong some things that an American teacher would say are correct. US Did you do your homework yet? Brit. Have you done your homework yet? US I already ate. Brit. I’ve already eaten. In British English, ‘have got’ is often used for the possessive sense of ‘have’ and ‘have got to’ is informally used for ‘have to’. This is much less common in American English. Brit. I’ve got two sisters. US I have two sisters. Brit. I’ve got to go now. US I have to go now. There are a number of other minor grammatical differences. Vocabulary There are a lot of examples of different words being used in British and American English. Here are a few of the commonest. angry (Brit.) = mad (US) autumn = fall boot (of a car) = trunk chemist’s = drug store cupboard = closet flat = apartment lift = elevator nappy = diaper pavement = sidewalk petrol = gas/gasoline rubbish = trash tap = faucet trousers = pants There are British words which many Americans will not understand and vice versa. There are also words which exist in both British and American English but have very different meanings. Spelling There are also a number of different spelling rules between British English and American English. 1 Some words that end in ‘-tre’ in British English end in ‘-ter’ in American English. US theater, center Brit. theatre, centre 2 Some words that end in ‘-our’ in British English end in ‘-or’ in American English. US color, labor Brit. colour, labour 3 Some words are shorter in American English than in British English. US catalog, program Brit. catalogue, programme There are other minor spelling differences between British and American English. Source : http://learnenglish.britishcouncil.org/ar/grammar-reference/british-english-and-american-english

This is very, very easy to look up by yourself. Have a read of this: http://en.wikipedia.org/wiki/Comparison_of_American_and_British_English