A very long time ago (in internet terms, that is – 2010), I wrote a post about the difference between prescriptivism and descriptivism, a sometimes false dichotomy that nonetheless can serve as a basic model of two broad approaches to language use. Put simply:
Descriptivists describe how language is used (and they may infer rules from that data).
Prescriptivists prescribe how language should be used (and they may enforce rules based on authority, tradition, house style, logic, personal preference, etc.).
Despite what you’ll sometimes hear about the ‘usage wars’, it’s not a black and white scenario: the sides overlap. I’m descriptivist in principle, but as an editor–proofreader by trade I wear a prescriptive hat, ensuring that clients’ prose is consistently styled and accords with the current norms of standard English or whatever register is desired in a given context.