A module is a set of codes that extend Drupal features and functionality.
Drupal modules :
Core modules : These modules are included with the main download of Drupal, and you can turn on their functionality without installing additional software.
Contributed modules : These modules are downloaded from the Modules download section of drupal.org, and installed within your Drupal installation.
You can also create your own module "CustomModules" using PHP programming, and Drupal's module API.
