Slang varies a great deal between the U.S. and Britain, and there are noticeable differences in pronunciation and accent. Inside the U.S. spelling and grammar are highly standardized, but pronunciation varies by region and social factors (such as race and education). American English derived from British English, but since independence in 1776 has diverged in various relatively small ways. All the major dictionaries published in the U.S. feature American English, often with a note on British differences.
Since 1945 Hollywood has overwhelmed the much smaller film industries in Britain and Canada, familiarizing the British with American English.
In spoken form, American English consists of a number of different dialects. Distinctive dialects of American English include those spoken in the South, New England, and the Midwest, as well as local areas such as New York City, to name only a few. Even these dialects can be broken down further into distinctive dialects, such as the English spoken in Brooklyn, or that spoken in Boston. African American English is itself a dialect.
The Internet has largely erased regional variations, so that the many blogs, twitters and facebooks seem to all reflect one standard language.
The use of English in Colonial America was established as a result of British settlement of the Thirteen Colonies. The first large wave of English-speaking settlers arrived in Virginia and New England in the 1607-1660 period. Other waves followed--and indeed immigrants from many other English-speaking countries are still arriving.
In the South the colonists came mostly from the western part of England, and the distinctive accents and dialects of that region shaped the southern accent.
By contrast the migrants to New England (now called Yankees) came from the east of England, and brought along different accents and dialects.
In the 18th century the Scots arrived, from Scotland and Ulster (Northern Ireland), bringing yet another dialect and a distinctive accent.
During the colonial era, there were also some speakers in North America of Dutch, French, German, Spanish, and Swedish, as well as numerous Native American languages. To a small extent, they affected American English, chiefly in the addition of some words and place names
By 1800 there were three main dialect areas, with slightly different pronunciations and much the same vocabulary: Northern (New England, New York and due west), Southern (Virginia to Georgia, and due west), and Midland (Pennsylvania and the lower Midwest). As the map shows, the same basic geographical patterns continue into the 21st century.
With the arrival of radio in the 1920s, the Midland version became standard American pronunciation.
Noah Webster not only compiled the first and most influential American dictionary, he also wrote a "Blue Backed Speller" that sold millions of copies and was used by most school children in the U.S. from the 1780s to the 1840s. Webster's goal was to provide a uniquely American approach to training children. His most important improvement, he claimed, was to rescue of "our native tongue" from "the clamor of pedantry" that surrounded English grammar and pronunciation. He complained that the English language had been corrupted by the British aristocracy, which set its own standard for proper spelling and pronunciation. Webster rejected the long-standing notion that the study of Greek and Latin must precede the study of English grammar. The appropriate standard for the American language, argued Webster, was "the same republican principles as American civil and ecclesiastical constitutions," which meant that the people-at-large must control the language; popular sovereignty in government must be accompanied by popular usage in language. "The truth is general custom is the rule of speaking—and every deviation from this must be wrong." 
In some ways, compared to British English, American English differs in its grammar, phonology and vocabulary. Many of these differences were amplified by separate attempts in both Britain and the United States to standardize English usage. For example, Webster, and his Webster's dictionary, was influential in firmly establishing many of the American spellings now in use today. Webster's dictionaries redefined Americanism within the context of an emergent and unstable American cultural identity. Webster's identification of his project as a "federal language" shows his competing impulses towards conservatism or regularity and innovation. The contradictions of Webster's project comprised part of a larger dialectical play between liberty and order within Revolutionary and post-Revolutionary political debates.
During his presidency, 1901-1909, Theodore Roosevelt failed in efforts to simplify the spelling of 300 common words. He (and other spelling reformers) wanted to replace "dropped" and "chased" with "dropt" and "chast", and replace "through" with "thru" and "thoroughly" with "thoroly." He tried to force the federal government to adopt the system by ordering the Public Printer to use the system in all public documents. The reform annoyed the public, forcing him to rescind the order. Roosevelt explained, "I could not by fighting have kept the new spelling in, and it was evidently worse than useless to go into an undignified contest when I was beaten. Do you know that the one word as to which I thought the new spelling was wrong — thru — was more responsible than anything else for our discomfiture?" Next summer Roosevelt was watching a naval review when a launch marked "Pres Bot" chugged ostentatiously by. The President waved and laughed with delight.
The Chicago Tribune also embarked on its own effort to simplify spelling and make it more phonetic.
Today, most people (both Americans and non-Americans) consider American English to be simpler and more phonetic than British English. Some others (especially native English speakers outside North America) regard British English as exclusively proper English; however, linguists dismiss this view as nonsense. In fact, both the British and American varieties are widely used today.
Americans have a more open, less formal society that has enabled them to coin and invent tens of thousands of new words and phrases, especially those that are viewed with disgust by many non-Americans; linguists and lexicographers have a herculean job to keep up, as the great compilations of Mathews (1951) and Cassidy (1985 - ongoing) demonstrate. The North American Regional Vocabulary Survey collected lexical data from the 1990s to 2004 from regions throughout the U.S. and Canada.
Some people regard the development and subsequent use of American English as a testament to the silliness of American society.
- American English and British English" in Concise Oxford Companion to the English Language ed. by Tom McArthur. (1998) online at many libraries
- Cassidy, Frederic Gomes, and Joan Houston Hall, eds Dictionary of American Regional English (4 vol 1985- ), A through Sk; final vol 5 is awaited; monumental coverage of the spoken and written language excerpt and text search vol 1
- Dillard, Joey Lee. Toward a Social History of American English (1985) excerpt and text search
- Garner, Bryan A. Garner's Modern American Usage (2nd ed. 2009) excerpt and text search
- Mathews, Mitford McLeod. A Dictionary of Americanisms on Historical Principles (1951), 1960pp; monumental dictionary of new words added by Americans
- Mencken, H. L. The American Language (1936) excerpt and text search of abridged edition
- "Color," "labor", "defense" and bank "check" are standard in the U.S.; "colour," "labour," "defence" and bank "cheque" are standard in Britain and the former British Empire.
- Michael Montgomery, "Voices of My Ancestors: a Personal Search for the Language of the Scotch-Irish," American Speech 2005 80(4): 341-365 in EBSCO
- See "A National Map of the Regional Dialects of American English (1997) and [http://www.geocities.com/yvain.geo/dialects.html "Dialect Map of American English "]
- See full text at Noah Webster, The American spelling book: containing the rudiments of the English language (1827) 168 pages
- Joseph Ellis, After the Revolution: Profiles of Early American Culture (1979) p 172.
- Full text online at Noah Webster, An American dictionary of the English language (1846), 1079 pages
- Henry F. Pringle, Theodore Roosevelt (1932), pp 465-7; H.W. Brands, T.R.: The Last Romantic (1998) pp555ff
- Charles Boberg, "The North American Regional Vocabulary Survey: New Variables and Methods in the Study of North American English," American Speech 2005 80(1): 22-60 in EBSCO