Differences Between American and British English

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" however, there are certainly preferences in use. The three major differences between 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

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 spellings 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), this is of course not always easy - or possible. The following guide is meant to point out the principal differences between these two varieties of English.
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 your finished your homework yet? OR Did you finish your homework yet?
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. Example He's gotten much better at playing tennis. British English - He's got much better at playing tennis.
Vocabulary
Probably the major differences between British and American English lies 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)
Rubber: (American English - condom, British English - tool used to erase pencil markings)
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
  

Once again, your dictionary should list whether the term is used in British English or American English.
For a more complete list of the vocabulary differences between British and American English use this British vs. American English vocabulary tool.
Prepositions
There are also a few differences in preposition use including the following:

• American English - on the weekend
  British English - at the weekend
  
• American English - on a team
  British English - in a team
  
• American English - please write me soon
  British English - please write to me soon
  

Past Simple/Past Participles
The following verbs have two acceptable forms of the past simple/past participle in both American and British English, however, the irregular form is generally more common in British English (the first form of the two) and the regular form is more common to American English.

• Burn
  Burnt OR burned
  
• Dream
  dreamt OR dreamed
  
• Lean
  leant OR leaned
  
• Learn
  learnt OR learned
  
• Smell
  smelt OR smelled
  
• Spell
  spelt OR spelled
  
• Spill
  spilt OR spilled
  Spoil
  spoilt OR spoiled
  

Spelling
Here are some general differences between British and American spellings:

Words ending in -or (American) -our (British) color, colour, humor, humour, flavor, flavour etc.
Words ending in -ize (American) -ise (British) recognize, recognise, patronize, patronise etc.

The best way to make sure that you are being consistent in your spelling is to use the spell check on your word processor (if you are using the computer of course) and choose which variety of English you would like. As you can see, there are really very few differences between standard British English and standard American English. However, the largest difference is probably that of the choice of vocabulary and pronunciation.