Drupal is an award-winning open-source Content Management System. It's a modular system, with an elegant hook-based architecture, and great code. Modules are plugins for Drupal that extend, build or enhance Drupal core functionality.
In Drupal 7 Module development book, six professional Drupal developers use a practical, example-based approach to introduce PHP developers to the powerful new Drupal 7 tools, APIs, and strategies for writing custom Drupal code.