Linguistics is descriptive; linguists describe and explain features of language without making subjective judgments on whether a particular feature is "right" or "wrong". This is analogous to practice in other sciences: A zoologist studies the animal kingdom without making subjective judgments on whether a particular animal is better or worse than another.
Prescription, on the other hand, is an attempt to promote particular linguistic usages over others, often favouring a particular dialect or "acrolect". This may have the aim of establishing a linguistic standard, which can aid communication over large geographical areas. It may also, however, be an attempt by speakers of one language or dialect to exert influence over speakers of other languages or dialects (see Linguistic imperialism). An extreme version of prescriptivism can be found among censors, who attempt to eradicate words and structures that they consider to be destructive to society.