Drupal

Note; oldish, reworking.

Drupal started out in 2001 as a forum CMS, but has evolved into a strong middle-of-the-road CMF (GPLv2). Its framework benefits from a well-balanced module, development ecosystem. There are a variety of basic levels of abstraction, with fields, entities, bundles for content, and hooks pretty much all the way down that other modules and themes can work with, building a network of possibilities. Bunches of module state+configuration can be saved as a Feature, and make files and install profiles are used to build and provision new sites.

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Sites Made with Drupal

Overview

Drupal CMS Benefits - The features of Drupal.

"Drupal is a publishing platform created by our vibrant community and bursting with potential. Use as-is or snap in any of thousands of free designs and plug-ins for rapid site assembly. Developers love our well-documented APIs. Designers love our flexibility. Site administrators love our limitless scalability. Drupal's content management features make it easy to create and manage your site."

The Drupal Learning Curve: a configurators view - A good overview of techniques and modules Drupal users will most likely learn over time.

What you can do with Drupal from an administration perspective: YouTube: Summary of Site Building with Drupal

Development

Modules (as can themes) use a hook API.

See also: Dev

Provision

Drupal deployments & workflows with version control, drush_make, and Aegir - Using Drush, the Drupal cli management/scripting tool, and Aegir, the Drupal based development and hosting system.
Using Drush Make - building a base Drupal installation through automation