Interface design deals with the process of developing a method for two (or more) modules in a system to connect and communicate. These modules can apply to hardware, software or the interface between a user and a machine. An example of a user interface could include a GUI

Gui

Gui or guee is a generic term to refer to grilled dishes in Korean cuisine. These most commonly have meat or fish as their primary ingredient, but may in some cases also comprise grilled vegetables or other vegetarian ingredients. The term derives from the verb, "gupda" in Korean, which literally...

, a control panel for a nuclear power plant , or even the cockpit of an aircraft.

In systems engineering, all the inputs and outputs of a system, subsystem, and its components are listed in an interface control document

Interface Control Document

An interface control drawing or interface control document in systems engineering and software engineering, describes the interface of interfaces to a system or subsystem.- Overview :An ICD may describe...

 often as part of the requirements of the engineering project.

The development of a user interface

User interface

The user interface, in the industrial design field of human–machine interaction, is the space where interaction between humans and machines occurs. The goal of interaction between a human and a machine at the user interface is effective operation and control of the machine, and feedback from the...

 is a unique field. More information can be found on the subject here:
User interface design

User interface design

User interface design or user interface engineering is the design of computers, appliances, machines, mobile communication devices, software applications, and websites with the focus on the user's experience and interaction...