British English Vs. American English British English and American English began to diverge in 1607, with the founding of the first American colony. Some of the major ways in which these two dialects have deviated include pronunciation, vocabulary, spelling, idioms, and the formatting of dates and numbers.  Of these differences, most are mutually intelligible by speakers of both dialects (i.e., ?colour? versus ?color?), but the idioms, and some more divergent vocabulary items, can often be elusive.
Hundreds of the more perplexing variations between British and American English are listed across the Internet. Most are single vocabulary words, such as ?draughts? as compared to ?checkers.? Many, however, are idioms ? for example, ?Bob?s your uncle? as opposed to ?done deal.? The vast majority of words and phrases such as these would not be known or understood by speakers of the other major English dialect.
As for mutually intelligible differences, spelling is probably the easiest for speakers of either dialect to understand. One theory is that spelling has changed so uniformly due to the fact that is was done intentionally by Americans ? around the time of the American Revolution ? for the purpose of distinguishing American English from British English.  Typical spelling differences are as follows:
American English British English Example
-e- -ae encyclopedia/encyclopaedia
-or -our color/colour
-ize -ise organize/organize
-er -re theater/theatre 
A noticeable difference between British and American English which is often good naturedly laughed at by speakers of both dialects is pronunciation. Specific differences in British and American pronunciation largely involve the following letters and letter combinations:
Letter(s) American Pronunciation British Pronunciation
a often short and flat usually long and rounded
t (mid-word) often reduced to ?d? or dropped fully annunciated
-ing ?g? is often dropped fully annunciated
u ?oo? ?ew?
-ter/-tor fully annunciated or ?t? dropped often ?t? and ?r? dropped 
Lastly, the formatting of dates and times is often different in both the written and spoken versions of British and American English. In British English, times are conveyed on a 24-hour system, which is known as ?military time? and used primarily in military contexts in the US. Common civilians (and off-duty military personnel) typically refer to time on a 12-hour system, utilizing ?a.m.? and ?p.m.? to differentiate between morning and evening times. Concerning dates, British English would specify that today?s date must be written ?20/5/2011,? while American English would swap the day and the month, making it ?5/20/11.?  This can create much confusion if it is not absolutely clear which system is being used.
When teaching English as a foreign language, it is valuable to know both which major dialect your students
have had the most exposure to, as well as which of the varieties of English is going to be most useful for them in the future, so that you can teach aspects of the dialect that will help them the most. This will make casual and formal conversation in English more natural for the students
after having have completed the course, as well as enable them to produce more professional written communication. Furthermore, learning the nuances of the dialect that will help them meet their English communication goals can help students
avoid embarrassing mistakes (such as asking an American coworker to pass them a ?rubber? ? which means ?condom? in American English; the student
should have asked for an ?eraser? instead!) With correct knowledge of students
? prior exposure to English dialects, and a focus on their eventual goals, English instruction can be catered to the needs of the class to enable optimum success.