Drupal Core

A lightweight framework makes up the Drupal core. This is what you get when you download Drupal from drupal.org. The core is responsible for providing the basic functionality that will be used to support other parts of the system. The core includes code that allows the Drupal system to bootstrap when it receives a request, a library of common functions frequently used with Drupal, and modules that provide basic functionality like user management, taxonomy, and templating, as shown below:

An overview of the Drupal core (not all core functionality is shown)

Note: The core also includes the basic functional building blocks for most web sites, including feed aggregation, blogging, polls, and forums.

Administrative Interface
The administrative interface in Drupal is tightly integrated with the rest of the site. All administrative functions are easily accessible through an administrative menu that appears at the top of the page when you are logged in as a site administrator. 

Drupal Modules : Extend the power of Drupal.

Drupal is a truly modular framework. Functionality is included in modules, which can be enabled or disabled. Features are added to a Drupal web site by enabling existing modules, installing modules written by members of the Drupal community, or writing new modules. In this way, web sites that do not need certain features can run lean and mean, while those that need more can add as much functionality as desired. This is shown below:

Enabling additional modules gives more functionality.

Modules can extend Drupal by adding new content types such as recipes, blog posts, or files, and behaviors such as e-mail notification, peer-to-peer publishing, and aggregation. Drupal makes use of the inversion of control design pattern, in which modular functionality is called by the framework at the appropriate time. These opportunities for modules to do their thing are called hooks.