An operating system (OS) is a program that allows you to interact with the computer -- all of the software and hardware on your computer.

Basically, there are two ways:
1. With a command-line operating system (e.g., DOS), you type a text command and the computer responds according to that command.
2. With a graphical user interface (GUI) operating system (e.g., Windows), you interact with the computer through a graphical interface with pictures and buttons by using the mouse and keyboard.

With Unix or Linux you have in general the option of using either command-lines (more control and flexibility) or GUIs (easier).