Varieties of English

rainbow

All languages vary. The number of speakers there are, the range of contexts in which the language is used and the geographical spread of the language will determine just how much variation occurs.
(The term 'variety' is preferred to such terms as accent, dialect, sociolect, idiolect and so on because they are difficult, notoriously, to define.)

Before we go on, take a little test and then click to reveal the answers.

Approximately how many users of English as a first language are there worldwide?
In how many countries is English spoken as an official language?
How many different standard forms are recognised?

It depends a little on how you count but a figure of between 375 and 400 million is the usual statistic. Approximately the same number speak English as a second language (often in official or occupational environments) and roughly 750 million speak it as a foreign language.
In roughly 60 countries and there are others, such as Israel, where the language has no official status but is widely used. Click to go to the Wikipedia list.
It depends. How do you define 'standard'? By most definitions, however, there are at least 12. Included in that list would be British, US, South African, Canadian, Singaporean, Hong Kong, Australian, New Zealand, Caribbean, Indian, East African and more Englishes.

How does language use vary?

There are a number of factors at work. Pause for a minute and think of two different factors which affect the sort of language we use. Click when you have an answer.

Here's the overview and some notes:

variety overview

There are two main factors to consider:

Who is using the language?: Language use will vary depending on where people come from and their social class.
What is the language being used for?: Language use will vary by registers. Occupational registers include things like jargon and special forms of English (legalese, IT-speak, medical language and so on) and topic-based registers will include things like sports registers, special hobby registers (can you read a knitting pattern?) and so on.

Some examples

Dialect

This is not the place to enter into the argument about whether a dialect is a language or not. If you want to know whether a dialect qualifies as a language, check the Ethnologue site.
Clearly there are many differences. What the United States calls a hood, the British call a bonnet, what Canadians call an automobile or car, Indians might refer to as a motor, what the British call a dunce might be called a bubu in Jamaica, what in parts of the USA are referred to as britches would be pants or trousers elsewhere and so on and on and on.
There are grammatical variations, too. In the USA the question might be
    Did you just get here?
    but in Britain, that would probably be
    Have you just got here?
In Southern England, you was is common and in the north, I were is similarly frequently heard. For more on the grammatical differences between British and American English, see the answer to a question here.
Spelling varies. The differences between BrE spelling and AmE spelling are mostly traceable to the reforming zeal of the American lexicographer, Webster. In general, AmE spelling is simpler and more consistent. That's not the same as saying that it's any more regular, of course.
Pronunciation varies regionally, too, of course. Everywhere.
If you are interested in the major differences between British and American pronunciation, see the answer to a question here.

Sociolect

This refers to the way that language varies by social class.
This is also not the place to enter the argument about how class is (or even whether it should be) defined. Suffice it to say that it is unlikely that you will hear the British royal family say Gis the salt, mate or find Austria pronounced as Orstria or One wonders where one's drill has got to on a building site. Grammar varies here, too, with a noted preference in some classes to use we was instead of we were and ain't instead of isn't and so on.
Pronunciation can vary markedly with social class. For example, pronouncing the 'r' in father is considered high status in some varieties (such as Southern USA) but low status in others (such as Britain and the American East Coast).

Accent

This is the most obvious form of language variation and doesn't need exemplifying here.

Style

There is separate guide on this site to style and register but we should note here that the social setting in which the user of a language happens to be will often determine how much subject-specific language, dialect, accent and slang are appropriate.

English as an International Language

It is a truth not widely enough recognised that most learners of English are not learning the language to speak to you or even to people like you. Most learners want English as a means of international communication (this is often called ELF: English as a lingua franca).
There are around half a billion websites out here. What percentage are in English? Click when you have guessed.

website languages

Source Wikipedia

Websites in languages other than English are increasing rapidly but the figures are still startling.
It isn't just out here on the web that English is dominant. In 1880, 36% of scientific publications were in English. That had risen to 64% by 1980. By 2000, among journals recognised by Journal Citation Reports, 96% were in English. Some researchers are beginning to complain quite loudly that in order to be published in prestigious journals, one must not only write in English, one must accept prevailing Anglo-American theoretical frameworks.
Here are some more startlers from the British Council's website:

English is the main language of books, newspapers, airports and air-traffic control, international business and academic conferences, science, technology, diplomacy, sport, international competitions, pop music and advertising
over two-thirds of the world's scientists read in English
three quarters of the world's mail is written in English
eighty per cent of the world's electronically stored information is in English
of the estimated 200 million users of the Internet, some thirty-six per cent communicate in English

Implications

Think for a moment about what the implications are for English Language Teaching and then click for a short list.

We should avoid (or at least be careful about) teaching regionally specific language.
We should know what our learners need English for. Occupational and topic registers are important here.
We should use language in the classroom which is not heavily influenced by our own dialect and accent. If that means learning to speak without such influences, so be it.
We should avoid teaching language which is confined to specific class- or topic-influenced settings. If that means cautioning students not to use non-standard grammar such as gonna or ain't, so be it.
We should expose our students to the main Englishes they are likely to encounter outside our classrooms.
We should be careful about the kinds of materials we use in class. If a song or other authentic text contains instances of non-standard or regionally-influenced grammar or lexis we should consider whether we should be using it and if need be warn the learners not to use it as a model.

Try a short test to make sure you have the essentials right.

References:
The British Council, http://www.britishcouncil.org/learning-faq-the-english-language.htm [accessed December 2013]
Wikipedia (http://en.wikipedia.org/wiki/Languages_used_on_the_Internet) [accessed December 2013]