Interface ?

Interface: It is a "existing entity" layer between the functionality  and consumer of that functionality. An, interface by itself is doesn't do anything. It just invokes the functionality lying behind.

Now depending on who the user is there are different type of interfaces.

Command Line Interface(CLI) commands are the existing entities, consumer is the user and functionality lies behind.

functionality: my software functionality which solves some purpose to which we are describing this interface.

existing entities: commands

consumer: user

Graphical User Interface(GUI) window,buttons etc.. are the existing entities, again consumer is the user and functionality lies behind.

functionality: my software functionality which solves some purpose to which we are describing this interface.

existing entities: window,buttons etc..

consumer: user

Application Programming Interface(API) functions or to be more correct, interfaces (in interfaced based programming) are the existing entities, consumer here is another program not a user. and again functionality lies behind this layer.

functionality:  my software functionality which solves some purpose to which we are describing this interface.

existing entities: functions, Interfaces(array of functions).

consumer: another program/application.