Descriptive linguistics is the work of analyzing and describing how language is spoken (or how it was spoken in the past) by a group of people in a speech community. All scholarly research in linguistics is descriptive; like all other sciences, its aim is to observe the linguistic world as it is, without the bias of preconceived ideas about how it ought to be. Modern descriptive linguistics is based on a structural approach to language, as exemplified in the work of Bloomfield and others.
Linguistic description is often contrasted with linguistic prescription, which is found especially in education and in publishing. Prescription seeks to define standard language forms and give advice on effective language use, and can be thought of as the attempt to present the fruits of descriptive research in a learnable form, though it also draws on more subjective aspects of language aesthetics. Prescription and description are essentially complementary, but have different priorities and sometimes are seen to be in conflict.
Accurate description of real speech is a difficult problem, and linguists have often been reduced to approximations. Almost all linguistic theory has its origin in practical problems of descriptive linguistics. Phonology (and its theoretical developments, such as the phoneme) deals with the function and interpretation of sound in language. Syntax has developed to describe the rules concerning how words relate to each other in order to form sentences. Lexicology collects "words" and their derivations and transformations: it has not given rise to much generalized theory.
An extreme "mentalist" viewpoint denies that the linguistic description of a language can be done by anyone but a competent speaker. Such a speaker has internalized something called "linguistic competence", which gives them the ability to extrapolate correctly from their experience new but correct expressions, and to reject unacceptable expressions.
There are tens of thousands of linguistic descriptions of thousands of languages that were prepared by people without adequate linguistic training. Prior to 1900, there was little academic descriptions of language.
A linguistic description is considered descriptively adequate if it achieves one or more of the following goals of descriptive linguistics: