The difference between a prescriptive point of view and a descriptive point of view is the difference between saying what should be and what is. A prescriptive point of view on language tries to establish the proper way to speak a language. A descriptive point of view seeks only to describe how the language is spoken.
In many places at many times, people have felt that there is a form of their language that is better than other forms of the language. People have felt that some ways of using language are more proper than others. For example, in America today, there are many people who do not like the way in which others use the words “your” and “you’re” incorrectly. The word “your” is supposed to be possessive, but many people use it in phrases like “your welcome” when they ought to use “you’re,” which is the contraction for “you are.” People who decry this sort of misuse of the language are looking at language from a prescriptive point of view. They feel that some usages are proper and others are not. They are saying what should be, not just what is.
On the other side of the discussion are people who do not think that linguists should be involved in finding the proper form of a language. These people feel that the role of the linguist is simply to describe the ways in which people actually use a language. In this view, all forms of a language are equally legitimate and so there is no reason to try to identify the “right” way to speak any language.
Thus, the prescriptive point of view is concerned with saying how a language should be spoken while the descriptive point of view is only concerned with documenting how the language is spoken by various people.