We discuss how particular words, phrases, and syntactic forms are
used; how they originated; and where in the English-speaking world
they're prevalent. (All this is called "description".) We also
discuss how we think they *should* be used ("prescription").
The last sentence reflects a point of view that has been somewhat controversial in AUE, that prescriptivists do not lay down rules about English usage, but only say how they think the language should be used. Another point of view is that 'prescriptivists' prescribe; that is, they lay down rules for proper English usage. That concept of 'prescriptivist' is fully in accord with the meaning of the word 'prescribe', which is (from Random House Webster's Unabridged Electronic Dictionary):
1. to lay down, in writing or otherwise, as a rule or a course of action to be followed; appoint, ordain, or enjoin.
3. to lay down rules; direct; dictate.
Defenders of the definition that says prescriptivists only recommend certain usages say that the meaning of 'prescribe' does not apply to the meaning of 'prescriptivism'. Opposed to that point of view is the definition of 'prescriptivism' in OED2, which says that prescriptivism consists of laying down rules that are expected to be followed.
Also opposed is the following statement in The Oxford Companion to the English Language (page 286, under the heading 'Descriptivism and Prescriptivism'):
Prescriptivism is an approach, especially to grammar, that sets out rules for what is regarded as correct in language.