• descriptivism is the belief that the meaning of words is determined by usage.
• prescriptivism is the belief that there is a correct way to use words.

These aren't mutually exclusive--one might argue that the correct way to use a word is the same way as everyone else is using that word. However, in my experience these concepts are most relevant in situations where someone is arguing that common usage of a word is incorrect.

My opinion is that the purpose of words is communication, and that the most effective way to use words to communicate is to use words the way your audience uses them. One might make recommendations on how to use a word based on the audience the communicator is trying to reach.

However, typically, if the intended meaning of a word has actually been misunderstood, nobody knows it has happened and both the communicator and receiver of communication go on believing different things about what was communicated. When a receiver of communication "corrects" a communicator on their usage of a word, the only reason they are able to do that is because they understood the meaning the commuinicator intended. This sort of correction shuts down conversations and isn't productive.