Standard English, also known as Standard Written English or SWE, is the form of English most widely accepted as being clear and proper.
Publishers, writers, educators, and others have over the years developed a consensus of what standard English consists of. It includes word choice, word order, punctuation, and spelling.
Standard English is especially helpful when writing because it maintains a fairly uniform standard of communication which can be understood by all speakers and users of English regardless of differences in dialect, pronunciation, and usage. This is why it is sometimes called Standard Written English.
There are a few minor differences between standard usage in England and the United States, but these differences do not significantly affect communication in the English language.
from Common Errors in English Usage ~
What is an error in English?
The concept of language errors is a fuzzy one. I'll leave to linguists the technical definitions. Here we're concerned only with deviations from the standard use of English as judged by sophisticated users such as professional writers, editors, teachers, and literate executives and personnel officers. The aim of this site is to help you avoid low grades, lost employment opportunities, lost business, and titters of amusement at the way you write or speak.
But isn't one person's mistake another's standard usage?
Often enough, but if your standard usage causes other people to consider you stupid or ignorant, you may want to consider changing it. You have the right to express yourself in any manner you please, but if you wish to communicate effectively, you should use nonstandard English only when you intend to, rather than fall into it because you don?t know any better.
Does it oppress immigrants and minorities to insist on the use of standard English?
Language standards can certainly be used for oppressive purposes, but most speakers and writers of all races and classes want to use language in a way that will impress others. The fact is that the world is full of teachers, employers, and other authorities who may penalize you for your nonstandard use of the English language. Feel free to denounce these people if you wish; but if you need their good opinion to get ahead, you'd be wise to learn standard English. Note that I often suggest differing usages as appropriate depending on the setting: spoken vs. written, informal vs. formal; slang is often highly appropriate. In fact, most of the errors discussed on this site are common in the writing of privileged middle-class Americans, and some are characteristic of people with advanced degrees and considerable intellectual attainments. However you come down on this issue, note that the great advantage of an open Web-based educational site like this is that it's voluntary: take what you want and leave the rest. It?s interesting that I have received hundreds of messages from non-native speakers thanking me for these pages and none from such people complaining that my pages discriminate against them.
The following is from http://grammar.about.com/od/grammarfaq/a/standardenglish.htm
For some, Standard English (SE) is a synonym for good or correct English usage. Others use the term to refer to a specific geographical dialect of English or a dialect favored by the most powerful and prestigious social group. Some linguists argue that there really is no single standard of English.
The Ongoing Debate
It is in fact a great pity that the standard English debate is marred by the sort of conceptual confusions and political posturings (no matter how poorly expressed) . . .. For I think there are genuine questions to be asked about what we might mean by "standards" in relation to speech and writing. There is a great deal to be done in this respect and proper arguments to be made, but one thing is clear for sure. The answer does not lie in some simple-minded recourse to the practice of the "best authors" or the "admired literature" of the past, valuable though that writing is. Nor does the answer reside in "rules" for speech laid down by either the "educated" of any official body held to be able to guarantee spoken "correctness." The answers to the real questions will be found to be much more complex, difficult and challenging than those currently on offer. For these reasons they might be more successful.
(Tony Crowley, "Curiouser and Curiouser: Falling Standards in the Standard English Debate," in Standard English: The Widening Debate, edited by Tony Bex and Richard J. Watts, Routledge, 1999)