HCI is the study of how people interact with computers and technology, with the goal of making that interaction effective, efficient, and satisfying. It combines computer science with fields such as psychology, social sciences, communication, and graphic and industrial design, since a good interface has to account for the user's cognition, the task they're trying to do, and the physical and social context they're in — not just the underlying code.
A system is useful if it helps solve a real problem, while it is usable if it is easy to understand and operate. The two are related but separate: a usable system with no real purpose still isn't worth much, and a useful system that's painful to operate loses much of its value in practice. For example, a penguin-tracking app may be easy to use but not useful to an accountant who has no need for it, while a command-line accounting tool might be very useful to that same accountant but hard to use without training.
A washing machine can be confusing when one dial cycles through many temperature and wash settings in a single sequence. This makes it easy to overshoot the setting you want, since the control doesn't map directly to one function and the current option isn't clearly visible. A better design would use clearly labeled, separate controls for each setting — for example, one dial for temperature and one for cycle type — so users can select their choice directly instead of guessing their way through unrelated options.
A prototype is an early model of a product used to test its design before committing to full development. Testing one with real users can reveal problems and confusion — where they hesitate, take a wrong path, or fail to notice a control — before the final product is built. Asking "Do you like it?" only captures a vague opinion; it wouldn't show whether users could actually complete the task, where exactly they struggled, how long it took, or whether they'd remember how to do it again later.
Voice input is an example of interaction beyond a keyboard and mouse. It can benefit people who have difficulty typing or using their hands, as well as people who are situationally limited — like a driver who needs to keep their hands on the wheel. To test it, I would compare how quickly and accurately users complete the same task using voice input versus a keyboard, tracking completion rate, time on task, and errors rather than relying on opinion alone.
What is Tangible Interaction? — Interaction Design Foundation
What Is Human-Computer Interaction and How Does It Work? — Coursera
Usability — CS4760/CS5760, Michigan Technological University