Example: Imagine that you want to write a descriptive essay about your grandfather. You've chosen to write about your grandfather's physical appearance and the way that he interacts with people. However, rather than providing a general description of these aspects, you want to convey your admiration for his strength and kindness. This is your reason for writing the descriptive essay. To achieve this, you might focus one of your paragraphs on describing the roughness of his hands, roughness resulting from the labor of his work throughout his life, but you might also describe how he would hold your hands so gently with his rough hands when having a conversation with you or when taking a walk.
A key contrast is to be found between these two approaches. A descriptive grammarian would say that a sentence is “grammatical” if a native speaker of the language would produce that sentence in speaking. The descriptive grammarian would then try to describe how that sentence is produced through theorizing about the mental processes that lead up to the surface form. A prescriptive grammarian , on the other hand, would say that something is grammatical only if the surface form conforms to a set of rules that the grammarian believes should be followed in order for a certain grammar style is achieved. (Note that I have tried to emphasize that the descriptive grammarian hears a form and tries to describe the mental processes underneath the produced (spoken) form, while a prescriptive grammarian does not hypothesize about the mental grammar at all, but is merely concerned with ‘editing’ the surface form.)