Are these definitions of prescriptivism and descriptivism a good place to start for someone who wishes to know the differences in the two ways of thinking?

"Prescriptivism is based on the view that one variety of language is inherently superior to others and that this more highly valued variety should be imposed on the whole of a particular speech community."

"Descriptivism is based on the view that the assignment of a superior status to one variety of language is often arbitrary and is more likely to be the result of socio-economic factors than of intrinic linguistic factors."

From Keywords in Language and Literacy. By Ronald Carter
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Comments  (Page 13) 
On the one hand...

The difference between prescriptivism and descriptivism is one involving value judgements. In this sense a prescriptivist will perceive a particular variety of a language as "the" language with all other varieties perceived as inferior varieties or dialects. A descriptivist will argue that the standard is a dialect as much as any other variety and that all versions of a language are equally valid.

On the other hand...

Prescriptivism and descriptivism are complimentary. When you set out to investigate a language you have to try and discover its rules - that is the descriptive approach. As you as you have decided what the rules are and set then down you are taking a prescriptive approach.

The point is that all language is a convention. All conventions have rules. Saying what you think the rules may be about a particular variety of a language does not necessarily mean that you think those rules should apply to all varieties of that language.

So, whilst I do not believe that any variety of English is inherently superior to any other, if asked whether a particular form is correct in Standard English I will give my opinion.