In computer science, the humans who are ultimately going to use a computer program regularly are typically called users. The user is — or at least, should be — always the one in control of using an interactive system. Thinking, planning and decision-making are all jobs for the user, and in fact, users can sometimes get quite resentful if the system tries to take away one of these jobs!
Sometimes, the intended user for an interactive system is extremely general. The Google search engine is an interactive system, and it is used by virtually everyone who uses the internet. Other times, a system’s user-base is much more specific. For example, software designed to control a medical imaging machine is only likely to be used by doctors, nurses, and medical technicians. Knowing the capabilities of your users is often an essential aspect of designing effective interactive systems.