The General/UserInterface is the part of an application with which the user interacts.
For typical Mac OS X applications, the General/UserInterface will be a General/GraphicalUserInterface with windows and menus and such, but those aren’t the only kind of interfaces available.
Most UNIX tools use the General/CommandLineInterface, which you can access from the Terminal application. This is generally more complex for the user, but it can be useful when you need to provide a basic General/UserInterface for a tool that normally runs without interaction.
There are other kinds of interfaces available as well. Things like General/SpeechRecognition provide a General/UserInterface that can be appropriate in some situations.
General/AppleScript is also a sort of General/UserInterface.