Differences Between American and British English

While there are certainly many more varieties of English, American English and British English are the two varieties most used in the world

The three major differences between American and British English are:

Pronunciation - differences in both vowel and consonants, as well as stress and intonation

Vocabulary - differences in nouns and verbs, especially phrasal verb usage

Spelling - differences are generally found in certain prefix and suffix forms

Use of the Present Perfect

In British English the present perfect is used to express an action that has occurred in the recent past that has an effect on the present moment.

For example:

I've lost my key. Can you help me look for it?
 

In American English the following is also posible:

  I lost my key. Can you help me look for it?

In British English the above would be considered incorrect. However, both forms are generally accepted in standard American English. Other differences involving the use of the present perfect in British English and simple past in American English include already, just and yet.

British English:
I've just had lunch
I've already seen that film
Have you finished your homework yet?
American English:

I just had lunch OR I've just had lunch
I've already seen that film OR I already saw that film.
Have your finished your homework yet? OR Did you finish your homework yet?