The User Interface (UI) is the point of human-computer interaction and communication in a device. This can include display screens, keyboards, a mouse and the appearance of a desktop. It is also the way through which a user interacts with an application or a website.
The UI is the product. To the end-user, the backend complexity is irrelevant; if the UI is confusing or broken, the product is broken. A well-crafted UI reduces the learning curve and increases user efficiency.
The best UI is no UI. We strive for interfaces that are self-explanatory and disappear into the background. We value clarity over decoration. Every pixel must earn its place on the screen; if it doesn't support the user's task, it is noise.