Composer is one of the greatest things that has happened to PHP lately.
Composer is a tool for dependency management in PHP. It allows you to declare the dependent libraries your project needs and it will install them in your project for you.
Thanks to Composer Manager a contributed module, we can now take advantage of this great tool while working with Drupal 8.
I will list the required steps to include an external library within a Drupal 8 module and use Composer Manager to download them.