The Command Line Interface (CLI), sometimes also referred to as the Terminal or Shell. Technically, the Terminal is the application that is used to interface w/ the CLI and the Shell is the actual CLI. The CLI is a more direct way of accessing your computer, particularly UNIX based systems, of which Mac OS X is one, and often gives you more control over various aspects of the computer. It is also the most efficient and sometimes only way to access parts of the computer remotely and allows actions to be scripted.

More background info:

Here are a few concepts that will help you understand how the command line works. Once you have a mental model/understanding of this, it will make things easier:

These Youtube videos are good to help you get a feel/to follow along with:

Here’s a comprehensive tutorial that should take you from 0 to something in a breezy way:

Another comprehensive tutorial:

For a comprehensive tutorial in the style of Learn Python the Hard Way:

More Learning:

Reference