An OS must have at least one kind of user interface. Today there are two major kinds of user interfaces in use, the command line interface(CLI) and the graphical user interface(GUI). Right now you are most likely using a GUI interface, but your system probably also contains a command line interface as well.
Typically speaking, GUIs are intended for general use and CLIs are intended for use by computer engineers and system administrators. Although some engineers only use GUIs and some diehard geeks still use a CLI even to type an email or a letter.Examples of popular operating systems with GUI interfaces include Windows and Mac OS X. Unixsystems have two popular GUIs as well, known as KDE and Gnome, which run on top of X-Windows. All three of the above mentioned operating systems also have built-in CLI interfaces as well for power users and software engineers. The CLI in Windows is known as MS-DOS. The CLI in Max OS X is known as the Terminal. There are many CLIs for Unix and Linux operating systems, but the most popular one is called Bash.