Here are some select examples of differences found in American and British English. In the first list the words are completely different. In the second list the spelling is slightly different.

Selected words that are completely different:

Selected spelling differences between American and British English:

More important are the grammar differences, notably on the question of subject/verb agreement, where American English is much more strict that the two agree in technical number. For example, while in the US, they say, 'The group is...', in the UK, they say, 'The group are...'

And the meaning of words can be different altogether, of course. Speakers in the US, will usually use the word 'goverment' to refer to the whole aparatus of the state, while in the UK, the word 'government' refers strictly to the Cabinet, that is, the Prime Minister and his selected ministers.

The differences can also be less subtle. 'I need a fag,' is quite a common office phrase where I work in London, but I doubt many American office workers say this when they step out to smoke a cigarette.