The term prescriptive grammar refers to a set of norms or rules governing how a language should or should not be used rather than describing the ways in which a language is actually used.
Prescriptivism involves the laying down of rules by those claiming to have special knowledge of or feeling for a language. Prescriptive advice tends to be conservative, changes being regarded with suspicion if not disdain. Descriptivism involves the objective description of the way a language works as observed in actual examples of the language.