George Bernard Shaw is famously quoted as having said that the British and the Americans are two nations “separated by a common language”. In reality, there is no real difference in the application of the language in grammatical terms between American English and British English: sentences are constructed in an identical manner and the application of punctuation is the same.
The main area of difference is in the vocabulary and spelling used. There are many differences in these areas, not immediately apparent to a many people but afficionados of Hollywood movies will be able to identify a few of the more obvious differences without difficulty. For example:
gas (US) v petrol (UK)
soccer (US) v football (UK)
apartment (US) v flat (UK)
attorney (US) v lawyer (UK)
sidewalk (US) v pavement (UK)
If you are a writer considering writing either in the UK, the US or both, you will need to ensure you tailor your writing for the readership you are trying to target. The use of American vocabulary, and particularly spelling, in British publications is frowned upon and will immediately switch a reader off; that’s if it ever gets past an editor first. Likewise, use of UK-specific vocabulary will have a similar effect in the USA. It is far better to take account of this at the beginning, than be put in the potentially embarrassing position of having to rewrite your text.
Of equal importance when understanding the differences between American and British English vocabulary and word usage, is spelling. There are some significant differences which, if not taken into account by the writer, will highlight that you may not have fully investigated your target audience. Again, for example:
center (US) v centre (UK)
donut (US) v doughnut (UK)
favorite (US) v favourite (UK)
tire (US) v tyre (UK)
catalog (US) v catalogue (UK)
color (US) v colour (UK)
If you are looking to write for both markets, it is worthwhile acquainting yourself with more of these kind of differences. It will also be useful to get a hold of UK and US English dictionaries (Websters for the US and Oxford OED for the UK).
Use the internet to look for key differences and ways in which the language is developing on each side of the Atlantic. Since the UK and the US are separated by an ocean and several thousand miles, we will increasingly find that the dialects of British and American English will continue to diverge as the modern languages develop. It is also worth investigating the now apparent differences in English language usage between the generations as well, especially in the modern urban environments.