A dialect is the common way in which people within a certain group use language. The language may be unique to people of a particular area, of a particular ethnic group, or to people at different socioeconomic positions. Dialect includes vocabulary, the way in which words are pronounced, and the rhythm and speed with which language is used.
There may be hundreds of distinct dialects within a language. Although it is generally assumed that those who speak a language will be able to communicate with speakers of different dialects within their language, they can become so distinct that communication may be difficult among speakers of the same language. This extreme is unusual, and in most instances, people who speak the same language can communicate at least to some extent, though there may be some language barriers.
The general rule applied by linguists, people who study language, is that people are considered to be speaking the same language if they can communicate. If the speech is so different that communication is impossible, the usage has most likely shifted to a distinct language. The linguist Max Winreich explained the difference a little differently, saying, “ A language is a dialect with an army and a navy.”
Although dialects exist in practically every language, English provides perhaps the most extreme examples of how diverse a single language can become. The English spoken in the U.S. is distinctly different than that spoken in Britain, which is distinctly different than what is spoken in Canada and in other countries where English is the first language of most citizens. Within each country, however, there are even distinct ways of speaking.
As an example, in the U.S., dialects are noticeably different in the south, east, and north. They exist in even more extremes, however, with there being ones specific to a state. Within states, and sometimes even within individual counties, there may be moderate differences in language that could make the speakers’ language recognizable as belonging to a certain group.
At one time, linguists considered one dialect to be the standard of the language and others to be examples of improper use. Most experts now believe that there is no "proper" dialect of a language and view the distinct forms as having their own rules of grammar and vocabulary usage. This means that, while a person from one language grouping may consider a speaker’s usage improper, the speaker may be speaking correctly according to the rules of language within his or her own group.