Dialect: A dialect (from the Greek word διάλεκτος, dialektos) is a variety of a language used by people from a particular geographic area. The number of speakers, and the area itself, can be of arbitrary size. It follows that a dialect for a larger area can contain plenty of (sub-) dialects, which in turn can contain dialects of yet smaller areas, etc.

A dialect is a complete system of verbal communication (oral or signed but not necessarily written) with its own vocabulary and/or grammar.

The concept of dialects can be distinguished from:—

sociolects, which are a variety of a language spoken by a certain social class,
standard languages, which are standardized for public performance (e.g. written standard),
jargons, which are characterized by differences in vocabulary (or lexicon according to linguist jargon), and
slang.

Varieties of language such as dialects, idiolects and sociolects can be distinguished not only by their vocabulary and grammar, but also by differences in phonology (including prosody). If the distinctions are limited to phonology, one often uses the term accent of a variety instead of variety or dialect.

http://en.wikipedia.org/wiki/Dialect