A user interface is how a user interacts with a device. In order for a device to work it requires input and in order for it to be useful for us, should provide output. This two way communication is the basic idea for user interfaces. The user interface is how the human interacts with the machine, commonly known as human-computer interaction (HCI). HCI is a major area of research and as such there are many user interfaces in existence. There are guidelines that people can follow in order to create good user-interfaces but they are not always followed. The cognitive side of HCI is a degree level topic, however the results of HCI are part of the A-Level specification.

In order to provide input there has to be some form of input device. The most common is the mouse and the keyboard, however they are not the only way to interact with a computer. Consider a game controller or a microphone. Both are ways to interact with the machine and require some form of user interface.