Typically people think of user interfaces as the graphical component of a computer software or a mobile app.

In practice, everything we use involves a UI. When driving a car, the steering wheel and gear stick are the interface through which the driver controls the car. When cooking, the pans, the oven and the stove are the interface.

This is important to keep in mind, because Having-the-right-user-interface-is-instrumental-in-performing-a-task.