The Command Line Interface (CLI) is just a way of controlling your computer. When you use a Graphical User Interface (GUI), you communicate with your machine by moving a mouse, clicking, double-clicking, dragging, scrolling, etc. With the CLI you communicate by typing commands. Generally speaking, the CLI can do anything a GUI can do and more. Here’s a demo of both interfaces at once:
If you’re completely new to the command line, try this game I made. It’ll walk you through some of the most basic commands.
How Do I Get to the Command Line?
In Ubuntu, you can type Control-Alt-T. In OSX, type Alt-Spacebar and type ‘terminal’. On Windows, you can install Powershell or Cygwin; your mileage may vary.