Prescriptive grammar holds that a definitive set of rules exist which govern the use of a language. Accordingly, a believer in prescriptive grammar considers that there are things which are acceptable and "correct", and others which are not. Some believers in prescriptive grammar object to any changes in the grammatical form of the language over time.
Prescriptive grammar and native speakers
There is some debate about whether prescriptive grammar is appropriate when dealing with native speakers of the English language. As there are no national bodies charged with pontificating on the "rules" of any variety of English, it would seem a little strange to attempt to hold native speakers to such hypothetical rules. Consequently the language produced by native speakers may be more productively analysed under the term descriptive grammar.
Prescriptive grammar and language learners
It seems to be clear that language learners, on the other hand, will benefit from some form of prescriptive grammar as they want, and need, to be told what is acceptable and what is not.