A user interface, also called a "UI" or simply an "interface," is the means in which a person controls a software application or hardware device. A good user interface provides a "user-friendly" experience, allowing the user to interact with the software or hardware in a natural and intuitive way.
Nearly all software programs have a graphical user interface, or GUI. This means the program includes graphical controls, which the user can select using a mouse or keyboard. A typical GUI of a software program includes a menu bar, toolbar, windows, buttons, and other controls. The Macintosh and Windows operating systems have different user interfaces, but they share many of the same elements, such as a desktop, windows, icons, etc. These common elements make it possible for people to use either operating system without having to completely relearn the interface. Similarly, programs like word processors and Web browsers all have rather similar interfaces, providing a consistent user experience across multiple programs.
Most hardware devices also include a user interface, though it is typically not as complex as a software interface. A common example of a hardware device with a user interface is a remote control. A typical TV remote has a numeric keypad, volume and channel buttons, mute and power buttons, an input selector, and other buttons that perform various functions. This set of buttons and the way they are laid out on the controller makes up the user interface. Other devices, such as digital cameras, audio mixing consoles, and stereo systems also have a user interface.
While user interfaces can be designed for either hardware of software, most are a combination of both. For example, to control a software program, you typically need to use a keyboard and mouse, which each have their own user interface. Likewise, to control a digital camera, you may need to navigate through the on-screen menus, which is a software interface. Regardless of the application, the goal of a good user interface is to be user-friendly. After all, we all know how frustrating it can be to use a device that doesn't work the way we want it to.