The user interface is the set of visual and interactive elements through which users interact with a system. Buttons, menus, icons, sliders, text fields, checkboxes, and gestures all belong to it. The interface is the surface between the user and the underlying code. It translates intention into action. When you tap a play button, the interface sends a command. When the video starts, the interface shows the result.

Interfaces appear on screens, but not only there. A car's dashboard is an interface. A thermostat dial is an interface. A voice assistant responds to spoken commands. The principles are similar across media. Elements should be visible, understandable, and responsive. Feedback matters. A button that does not react to a tap feels broken. Consistency matters too. If one screen uses a blue link and another uses green, the user hesitates.

Common UI elements

• Navigation bars and menus.
• Buttons, toggles, and sliders.
• Forms and input fields.
• Icons and visual indicators.

A good interface is not just attractive. It makes the next action obvious and confirms what just happened. Users should never wonder whether their tap registered.