Interface

An interface or shell is what the user uses^[1] on an operating system to make the computer do tasks, like writing a document, or loading a web page. There are two types of interfaces, a command line interface (CLI for short) - where people write commands to make the computer perform tasks, or a graphical user interface (GUI for short), where people use their Computer mouse to click on buttons to perform tasks.

1. ↑ Interfaces are not only for the end user (average computer user), everybody uses interfaces!