To get us started, let's take a look at the definition of a module:
Modules are plugins for Drupal that extend, build, or enhance Drupal functionality. Some core modules are required by Drupal in order to function, some are optional, and others can be obtained from the Drupal community.
It is by enabling certain modules that you will be able to achieve a diverse, and more importantly, functional site. As there are plenty of modules available, I can't hope to bore you with all the ins and outs of every single one. Instead, we will enable and briefly work with some of the more interesting and useful ones to demonstrate how the module system works and how to use it.
If there is a module required by your site that is not covered here, ...