American English

American English or US English is the dialect of the English language spoken in the United States of America. It is different in some ways from other types of English, such as British English. Most types of American English came from local dialects in England. During the 18th and 19th centuries, pronunciation changed less in America than in England.

Use 
Many people today know about American English even if they live in a country where another type of English is spoken. They hear and read American English through the media, for example movies, television, and the Internet, where the most common form of English is American English.

Because people all over the world use the English language, it gets many new words. English has been changing in this way for hundreds of years. For example, the many millions who speak Indian English frequently add American English words to go along with its British English base and many other words from the various Indian languages.

Sometimes people learn American English as it is spoken in the US. For example, in telephone call centers in India and other places, people often learn American English to sound more like their customers who call from the US. These people often keep using American English in everyday life.

Spelling 

There are many words that sound the same in both American and British English but have different spellings. British English often keeps more traditional ways of spelling words than American English.

Vocabulary 
There are also some words in American English that are a bit different from British English, e.g.:
 aeroplane is called "airplane"
 ladybird is called "ladybug"
 lift is called "elevator"
 toilet is called "bathroom", "restroom" or "comfort station"
 lorry is called "truck"
 nappies are called "diapers"
 petrol is called "gas" (or "gasoline")
 the boot of a car is called a "trunk"
 a dummy is called a "pacifier"
 trousers are called "pants"
 underground is called "subway"
 football is called "soccer"
 braces are "suspenders" ("suspenders" in British-English are a type of clothing worn around the lower leg to stop socks/sox from sagging, or around the upper leg by people wearing stockings)

Regional accents 
General American English is the kind most spoken in mass media. It more vigorously pronounces the letter "R" than some other kinds do. "R-dropping" is frequent in certain places where "r" sound is not pronounced after a vowel. For example as in the words "car" and "card" sounding like "cah" and "cahd". This occurs in the Boston area. 
Some regional accents of American English include

Appalachian English - This is the stereotypical hillbilly accent. This accent is completely rhotic and can even have phantom Rs (in words they don't belong)

General Southern - This is a range of accents which tend to be rhotic or semi-rhotic, have glide deletion (in which I is converted to broad A)

Tidewater English - A non-rhotic (r-dropping) southern variety that also has a "Scottish" or "Canadian" raising of the "ow" diphthong in words like "house" "about" "brown", etc.

Charleston and Savannah English - Almost extinct accents that are non-rhotic

Boston English (also east New England English) - This is the most famous non-rhotic American accent and what most other non-rhotic American varieties often get compared to. Other Bostonian features include limited Canadian raising of the "ow" diphthong (before voiceless consonants such as in words like "house" and "about").

New York City English - One of the most recognizable dialects in the US, NYC English is characterized by variable non-rhoticity or semi-rhoticity, a rounding of the long o sound ( making " coffee" and "thought" sound like "cawfee" and "thawt").

South Louisiana English - This group of non-rhotic accents can be heard in New Orleans and its surrounding areas and can be described as a combination between New York City English and Southern American English.

Northern Midwest English - The accents of this area tend to sound a lot like Canadian English.

Valley girl and surfer dude - This accent is common to southern California and has features like " vocal fry" (creaky voice), and "upturn" at the ends of sentences.

References

Other websites 

 American English -Citizendium