Prescriptivism & Descriptivism.

Definitions:

Prescriptivism...

"Prescriptivism is based on the view that one variety of language is inherently superior to others and that this more highly valued variety should be imposed on the whole of a particular speech community."

          Keywords in Language and Literacy. By Ronald Carter

"Based on or establishing norms or rules indicating how a language should or should not be used rather than describing the ways in which a language is used."

          American Heritage Dictionary

Descriptivism...

"Descriptivism is based on the view that the assignment of a superior status to one variety of language is often arbitrary and is more likely to be the result of socio-economic factors than of intrinsic linguistic factors."

          Keywords in Language and Literacy. By Ronald Carter

"Of or relating to the study or the description of a language or a specific stage of a language, with emphasis on constructing a grammar without regard to historical development, comparison with other languages, or advocated norms for correct or proper usage."

          American Heritage Dictionary

Well known prescriptivist's include...

• Alexis Manaster Ramer
• Lynn Truss
• Jean Aitchison
• Basil Bernstein
• David Crystal

David Crystal believes... 
"A prescriptive grammar is essentially a manual that focuses on constructions where usage is divided, and lays down rules governing the socially correct use of language. These grammars were a formative influence on language attitudes in Europe and America"

"A world of unchanging linguistic excellence... exists only in fantasy."

Well known descriptivist's include...

• Noam Chomsky 
• Frank Palmer
• Alexandra D’Arcy
• Leonard Bloomfield

Leonard Bloomfield aimed...

"to observe the linguistic world as it is, without the bias of preconceived ideas about how it ought to be, based on a structural approach to language including both syntax (e.g. the construction of phrases) and morphology (e.g. the construction of words)."