While there are certainly many more varieties of English, American English and British English are the two varieties that are taught in most ESL/EFL programs. Generally, it is agreed that no one version is "correct," but there are certainly preferences in use. 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 and the names of specific tools or items
• Spelling - differences are generally found in certain prefix and suffix forms

The most important rule of thumb is to try to be consistent in your usage. If you decide that you want to use American English, then be consistent in your spelling (i.e. "The color of the orange is also its flavour" - color is American spelling and flavour is British). Of course, this is not always easy or possible. The following guide is meant to point out the principal differences between these two varieties of English.

Minor Grammar Differences

There are very few grammar differences between American and British English. Certainly, the words we choose might be different at times. However, generally speaking, we follow the same grammar rules. With that said, there are a few differences. 

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 possible:
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 you finished your homework yet? OR Did you finish your homework yet?

Two Forms to Express Possession

There are two forms to express possession in English: have or have got.

Do you have a car?
Have you got a car?
He hasn't got any friends.
He doesn't have any friends.
She has a beautiful new home.
She's got a beautiful new home.

While both forms are correct (and accepted in both British and American English), have got (have you got, he hasn't got, etc.) is generally the preferred form in British English, while most speakers of American English employ the have (do you have, he doesn't have etc.)

The Verb Get

The past participle of the verb get is gotten in American English.

American English: He's gotten much better at playing tennis.

British English: He's got much better at playing tennis.

"Have got" is used predominately in British English to indicate "have" in the sense of possession. Strangely, this form is also used in the United States with the British participle "got," rather than "gotten." Americans will also use "have got to" in the sense of "have to" for responsibilities.

I've got to work tomorrow.
I've got three friends in Dallas.

Vocabulary

The largest differences between British and American English lie in the choice of vocabulary. Some words mean different things in the two varieties, for example:

Mean: American English - angry, bad humored, British English - not generous, tight-fisted.

American English: Don't be so mean to your sister!

British English: She's so mean she won't even pay for a cup of tea.

There are many more examples (too many for me to list here). If there is a difference in usage, your dictionary will note the different meanings in its definition of the term. Many vocabulary items are also used in one form and not in the other. One of the best examples of this is the terminology used for automobiles.

• American English - hood / British English - bonnet
• American English - trunk / British English - boot
• American English - truck / British English - lorry

Spelling

Here are some general differences between British and American spellings:

• Examples of words that end in -or in American English and -our in British English: color/colour, humor/humour, flavor/flavour
• Examples of words that end in -ize in American English and -ise in British English: recognize/recognise, patronize/patronise

The best way to make sure that you are being consistent in your spelling is to utilize the spell check tool associated with your word processor and select the type of English (American or British) you'd like to use.