What is user interface?

Let's say you have a database, filled with a large amount of data. The database was built in a program that is able to sort, retrieve, edit and otherwise manage the data. The part of the program that you can see, and where you enter commands, is the user interface. Another example would be the Windows desktop. Behind the scenes, millions of bytes of code tell the hardware what to do, and execute different tasks you ask them to. But the Graphical User Interface or GUI..that's your desktop, the stuff you see on your screen...is the user interface.