People who are paying your bills and salary. They can be your buyers, customers, clients, students, grant givers, sponsors, patrons.
We are used to direct buyer-user-payer understanding. With changing social and business environment and new types of businesses and earning possibilities, it might become a norm to separate these notions.
One of examples is blogging. People write blogs and other people read them. In most cases they read for free and the reader is user in this case. Many blogs live from advertising. Companies (or other bloggers) are paying to reach particular audience. They are buyers – they buy an audience. The same happens in many other case, for example with products that are bought as gifts, children products and many others. We have to think about both participants – user and the payer.