British English
British English is the form of English spoken in the United Kingdom.
American English is a variant of English that diverged from British English after the period of initial emigration from Great Britain to the United States and has since developed independently. There are many differences between British English and American English. In general these are not an impediment to understanding, but see the American and British English Differences. A literate, educated English speaker will generally know both forms. Due to the combined effect of the wide reach of US media and American insularity, knowledge of American English in Britain is more common than the reverse.
British and American English differ in words borrowed from foreign languages. British English includes many words from the languages of the Commonwealth while American English has words from its Spanish speaking neighbors and European immigrants.
For more on American borrowed words, see American English.