The term American English isn't an accurate term, but it is not an oxymoron as some librals would want you to believe
It is considered amongst scholars of linguistics not to be a language in itself, but rather a dialect of the original language spoken in Great Britain.
The use of English in the United States was inherited as a result of British colonization of the country which at the time was partly a British colony. The first wave of English-speaking settlers arrived in North America in the 17th century. During that time, there were also speakers in North America of Dutch, French, German, Spanish, Swedish, Scots, Welsh, Irish, Scottish Gaelic, Finnish, as well as numerous Native American languages. To a certain extent, the American dialect spoken commonly throughout the USA comes from the intertwining of many of these languages and dialects.
In many ways, compared to British English, American English is conservative in its phonology. Dialects in North America are most distinctive on the East Coast of the continent partly because these areas were in contact with England, and imitated prestigious varieties of British English at a time when those varieties were undergoing changes. Also, many speech communities on the East Coast have existed in their present locations longer than others. The interior of the United States, however, was settled by people from all regions of the existing U.S. and, as such, developed a far more generic linguistic pattern.
Examples of similar, albeit less conservative, English dialects are found throughout the world including Britain itself. In Singapore for example a dialect, similar to English is spoken by the countries inhabitants. The dialect, dubbed 'Singlish' by its users can be best described as an amalgamation of English, as spoken throughout the Empire at the time, and other South-East Asian languages, namely Mandarin Chinese. 'Singlish' is a vividly colorful language and one which the Singaporean people are, quite rightly, extremely proud of. It forms an aspect of the countries identity, much in the same way as dialects such as 'Cockney' characterise the East-End of London.
Dialects of the English language are also spoken widely throughout Africa and the West Indies (also former British colonies). These dialects have been labelled 'Pidgin-English' and again are an amalgamation of native languages mixed with that of the English speaking British colonials.
As mentioned previously, English dialects are found extensively throughout the United Kingdom and Northern Ireland. Cockney, which in its rhyming form is perhaps the most famous example is spoken rarely now in the East-End of London. This is not the only notable example however, 'Scouse', originating in Liverpool, is spoken in Liverpool. In addition to this, different regions will also adopt their own terminologies, these are often area exclusive but have extended into 'popular culture' through music and art, an example of which can be found in The Arctic Monkeys song 'Mardy Bum', essentially meaning a grumpy or sulky person. Despite the wide ranging usage of such dialects however, they are not recognised either by their users or by authoritative sources to be correct English. As such, if use of these dialects are found on applications or examination papers they will be sent back, or in the case of academia marked down.
This is not to say that use of dialects is wrong, indeed many would argue the opposite in that they provide color to a rapidly greying world. In accordance to this it can be argued that use of such dialects should be permitted on the internet and in the public sector, providing that others can understand and that the usage is not offensive. On the other extreme however, limiting an area, for example an internet domain, to one dialect in the name of conservatism, is abhorrent and arguably anti-intellectual. We can only be grateful that such an aggressive attack on our freedom of speech is not present in our fair nation.