Prescriptivism is the belief that Standard English and Received Pronunciation are the only correct ways of speaking; all regional dialects and variations are seen as improper or inferior

Descriptivism is the belief that all dialects and accents are of equal status and have no difference in terms of perceived intelligence or reliability etc. Opposite of Prescriptivism.

Prescriptivists include: Henry Fowler, Robert Lowth, Simon Heffer
Descriptivists include: Samuel Johnson, Leonard Bloomfield, Bertrand Russell

Dr. Samuel Johnston (1709-1784) believed that it would be irrational to see no change in what is viewed as correct language over the course of generations, and that it is fairly reasonable to assume changes will occur in language throughout time. He also argued that language regulators who attempted to preserve languages from being altered by others through creolisation etc. prevent language growth and are hardly beneficial.