Descriptive English and prescriptive English reflect two fundamentally opposing perspectives of the English language, the function of language and approaches to studying language. According to descriptive linguists and grammarians, English should be studied descriptively. Linguists and grammarians should undertake to study what English (language, more broadly) is, how it is used, what its forms are and how it functions in different contexts.
In contrast to descriptive English, prescriptive English is primarily considered with what English (or language, generally) should be, how it should be used, what its forms should be and how it should function. Prescriptive linguists approach language as a means of enforcing certain societal and syntactical standards of communication. Presciptivists adhere to classical traditions of language, aiming to preserve structured forms of language as sacred and whole.
Prescriptivists endow language (rather than the people that use language) with authority. Speakers and users of language are deprived of rights to manipulate language; rather, they are obligated to uphold conventional, structured standards of language. Problematically, structured language cannot enforce itself. Traditionally, grammarians and linguists, therefore, have assumed the right of authority to determine appropriate and inappropriate uses of language, writing dictionaries, thesauruses and other reference texts to provide the rules for "correct" language.
Descriptivists acknowledge language as an organic form, changing and adapting to meet the communicative needs of the people that use it. Therefore, the "rights" or authority are assigned to communicants. Descriptivists recognize slang and other violations of conventional grammar as valid forms. After all, descriptivists understand language as a medium, so there exist no correct and incorrect uses of language. Spelling, grammar, pronunciation and syntax are studied to help linguists understand how these features are being used and adapted, but these aspects of language are not used to promote or enforce standards.