Drupal Principles

• Modular and extensible. Drupal aims to provide a slim, powerful core that can be readily extended through custom modules.
• Quality coding. High quality, elegant, documented code is a priority over roughed-in functionality.
• Standards-based. Drupal supports established and emerging standards. Specific target standards include XHTML and CSS.
• Low resource demands. To ensure excellent performance, Drupal puts a premium on low-profile coding (for example, minimizing database queries). Drupal should also have minimal, widely-available server-side software requirements. Specifically, Drupal should be fully operational on a server with Apache web server, PHP, and either MySQL or PostgreSQL.
• Open source. Drupal is based on the open source philosophy of collaborative free software development and is licensed under the GPL. Drupal is itself open source and builds on and supports other open source projects. Specifically, Drupal is coded in the open source scripting language PHP and supports as primary data sources the open source database formats MySQL and PostgreSQL.
• Ease of use. Drupal aims for a high standard of usability for developers, administrators, and users.
• Collaboration. Drupal development supports open, collaborative information sharing systems and approaches.
  
  

As of version 5.*, Drupal comes up with the following core modules
Core (required):

• Block – Controls the boxes that are displayed around the main content.
• Filter – Handles the filtering of content in preparation for display.
• Node – Allows content to be submitted to the site and displayed on pages.
• System – Handles general site configuration for administrators.
• User – Manages the user registration and login system.
• Watchdog – Logs and records system events.

Core (optional):

• Aggregator – Aggregates syndicated content (RSS, RDF, and Atom feeds).
• Book – Allows users to collaboratively author a book.
• Comment – Allows users to comment on and discuss published content.
• Contact – Enables the use of both personal and site-wide contact forms.
• Drupal – Lets you register your site with a central server and improve ranking of Drupal projects by posting information on your installed modules and themes
• Forum – Enables threaded discussions about general topics.
• Legacy – Provides legacy handlers for upgrades from older Drupal installations.
• Menu – Allows administrators to customize the site navigation menu.
• Path – Allows users to rename URLs.
• Ping – Alerts other sites when your site has been updated.
• Profile – Supports configurable user profiles.
• Search – Enables site-wide keyword searching.
• Taxonomy – Enables the categorization of content.
• Tracker – Enables tracking of recent posts for users.
• Upload – Allows users to upload and attach files to content.

There are other useful contributed modules like:

• Content Construction Kit (CCK) which allows the administrator to create and customize fields using a web browser.
• CiviCRM – Constituent relationship management solution.
• Workflow – Workflow module allows the creation and assignment of arbitrary workflows to Drupal node types. Workflows are made up of workflow states.
• Views – The views module provides a flexible method for Drupal site designers to control how lists of content (nodes) are presented.