Not all people who speak a language speak it the same way. A language can be subdivided into any number of dialects which each vary in some way from the parent language. The term, accent, is often incorrectly used in its place, but an accent refers only to the way words are pronounced, while a dialect has its own grammar, vocabulary, syntax, and common expressions as well as pronunciation rules that make it unique from other dialects of the same language. Another term, idiolect, refers to the manner of speaking of an individual person. No two people's idiolects are exactly the same, but people who are part of the same group will have enough verbal elements in common to be said to be using the same dialect.

Three things are needed for a new dialect to develop: a group of people living in close proximity to each other; this group living in isolation (either geographically or socially) from other groups; and the passage of time. Given enough time, a dialect may evolve to the point that it becomes a different language from the one it started as. English began existence as a Germanic dialect called Anglo Saxon that was brought to England by invaders from Germany. The Anglo Saxon peoples in England were now geographically isolated from their cousins in Germany which allowed the dialects to evolve in different directions. Other invaders would also influence the development of English with their languages until the modern English we speak today has become so different from the modern German spoken in Germany that a speaker of one cannot understand a speaker of the other. Thus English and German are considered to be two distinct, though related, languages. The other modern languages in this family are Dutch, Swedish, Danish, Norwegian, and Icelandic.

This issue of mutual understandability is what in theory is used to determine what is a dialect and what is a language, but in reality there are social and political issues involved too. The government of a country might declare that all the languages spoken in that country are actually dialects of one language in order to create the illusion of polital unity, while the government of another country might declare that the dialect spoken by its people is actually a unique language from other countries that speak dialects of the same language in order to create a sense of national pride. History is full of governments that have tried to impose a single language on all of its people with varying results: sometimes the minority languages go entirely extinct, sometimes they are reduced to surviving only as dialects of the majority language, and sometimes new languages are unintentionally created by a blending of the two languages.

This brings us to three other language terms that are worth mentioning here. When two or more groups of people who speak different languages need to communicate with each other on a regular basis and do not want to actually learn each others' language (such as when the European merchants started trading with other peoples around the world), they may develop what is called a pidgin language. This is a simplified language that usually has as few words as possible in its vocabulary (taking some from both languages) and has been stripped of any fancier grammatical rules like the use of multiple verb conjugations and tenses - a kind of "Me Tarzan, you Jane" way of talking. A pidgin is nobody's native language and is used only in business settings. In fact, the word "pidgin" may be derived from the way Chinese merchants mispronounced the English word "business." However, in some cases, the children in one of these areas might grow up learning the pidgin as their first language. When this happens, the pidgin can grow in complexity into a creole language with a larger set of grammatical rules and a much larger vocabulary that share elements of all the languages that went into creating it.

Finally, jargon is a specialized vocabulary used by people within a particular discipline such as medical jargon for doctors, legal jargon for lawyers, or academic jargon for college professors. While jargon words occasionally filter up into a mainstream dialect, they are usually used only by experts and only when they are discussing their particular field. Critics argue, with some justification, that jargon needlessly complicates a statement that could be expressed in a more clear manner. Users of it argue, also with justification, that it is a more precise manner of speaking, although many examples can be found (especially in politics and business) where it has been used intentionally to obscure the fact that the speaker is trying to avoid being precise.

The modern development of communications technology may possibly slow down the evolution of dialects and languages. For the first time in history, a single dialect (sometimes called Network Standard) can be broadcast over an entire country, so very few people are actually living in geographic isolation anymore. However, the existence of racism, poverty, and class distinctions cause some groups to remain socially isolated from the mainstream of a culture, giving rise to social dialects like Black English (Ebonics) spoken by some African Americans in urban areas. There was recently a great deal of political controversy (ignoring the linguistic facts) over whether Ebonics should be considered a unique language, a "legitimate" dialect of English, or "illegimate" gutterspeak. Also, teenagers enjoy creating their own dialects that they can use to quickly determine who is or is not part of the "in crowd" and as a "secret language" in front of their parents. These dialects tend to go in and out of fashion very quickly; by the time an expression has filtered up to the mainstream dialect adults understand, the teenagers have moved on to something else. Even the Internet has given birth to what might be called a new social dialect (derived from hacker jargon) containing words like IMHO, IIRC, and ROTFLMAO.

Contrary to what your teachers probably tried to tell you, there is no such thing as "correct English." Any manner of speaking that is following the rules of a dialect is equally "correct." Words like ain't are "real" words in some dialects and perfectly acceptable to use. However, people are judged by the way they speak, and dialects carry different levels of social prestige with them based on the prejudices within a society. Generally, the southern dialects of American English carry a lower prestige, at least among northerners who will assume that a person speaking a southern dialect is less intelligent and less educated than they are. Some educated southerners even feel this way and will "correct" their speech to meet northern standards. The New York City dialect carries the lowest prestige of all (Received Standard, a dialect of British English used by the BBC and the royal family, carries the highest prestige - even among Americans). For this reason, schools try to rid children of the local dialects they learned from their family and friends in favor of a more prestigious one. (Of course, some sentences like, "Me are a educated person," would be incorrect in every dialect.)

American dialects come in many flavors. The map and list below show the major (and a few minor) geographic dialects and subdialects of English spoken in the United States. Many of these may be further subdivided into local subdialects that are not shown here. Obviously, the borders between dialect regions are not well defined lines, as a map like this would imply, but a gradual transition extending on both sides of the line. Also, as we enter the 21st century, many of the features described below have become much less prevalent than they were during the first half of the 20th century.