What is module in drupal?

A Module in Drupal is an optional plug-in that adds functionality to the core of the Drupal Content Management System (CMS). An example could be one of the many social plug-ins for this open source CMS, which allow you to have the social buttons on your web pages, such as the "like" button or "tweet" button.

There are many modules of varying complexity, some are dependent on other modules, and will require you to install those other modules alongside. The Drupal website has a section where you can search for all kinds of modules and it generally provides a good explanation as to the functionality and compatibility of each module.

It is generally recommended that you install models to the sites/all/modules folder of your Drupal installation, as this keeps your chosen "optional" modules separate from the core installation (which includes modules also) and makes it easier to update the CMS when the time comes.

Modules are installed separately and which one you choose depends on the functionality that you require.