Whare are the Drupal Components

Drupal has 3 types of components: 

Data Storage:

      The content of your site and its configuration settings are stored in a database. By default, each website has its own database. You can configure multiple sites to share a database as well. So, if you are planning multiple sites that need to share the same data, this is possible.

       On the topic of multiple sites sharing, you should also know that you can have multiple sites, each with its own database, sharing the same Drupal installation.

      For example, assume you provide Drupal train-ing and host the training sites. You can run all the training sites from one installation of Drupal. Each

training site would have its own database, but they would share the code that is Drupal.  Data in the database is only part of your storage needs. If you have any media fi les or documents that you want to upload to the site, Drupal stores those files, by default, in a directory on the server associated with the site. If you have sites sharing the Drupal installation, the files will be kept sepa-rate from each other unless the sites have been configured to do otherwise. The setting that tells Drupal where to save files is stored in the database for that site. This means a series of sites using the same installation of Drupal won’t get their media and documents mixed up.

Modules

If you have talked to anyone about Drupal, you probably heard him or her say something like,
“There is a module that will do that.”
Drupal is made up of a series of scripts that work together to produce the features you see in your
site via a browser. The most common scripts are referred to as modules. Modules are bundles of
code that enable Drupal, you, and/or your site visitors to see or do something in your Drupal site.
Drupal modules can be categorized. Here are four categories often referred to in the Drupal
community:

  Core 

     Core modules are required for Drupal to work. You can see these modules in the administrative module list, but you cannot disable them.

  Core optional

     Optional modules provide functionality that you can add to your site. Some of the most commonly used modules will already be enabled for you by default, although you can disable them if you do not need them. Each new version of Drupal refines the number of core modules, making the enabling of commonly used features easier.

  Contributed

   Contributed modules are pieces of code that you can add to enhance the      functionality of your site. Contributed modules are developed and maintained by members of the community. Some contributed modules are now part of the core 

  Custom

     Custom modules are pieces of code that you create to provide specific functionality for your site. Custom modules are maintained by you. You can contribute custom modules to the community, but unless someone wants to maintain the contribution for you, you will be responsible for its maintenance and support. If you have a particular feature you want to include in your plan, searching Drupal.org for existing modules that might support that feature is sometimes helpful. Planning a feature whose module does not yet exist could impact how the site is developed, when it will be finished, and the skills you
need on board to actually implement the site.

Themes

   A drupal theme refers what you see from the browser about website. How to display the content in specific order.