Drupal

Drupal is a modular framework and content management system that is written in a programming language called PHP. Just as many modern Content Management Systems do, Drupal allows a system administrator to add and organize the content, customize presentation, automate admin tasks, and manage viewers and contributors.

Drupal has a highly developed programming interface but a low programming system. It has acquired the title of "web application framework" because it is capable of enabling a wide range of services including content management and transactions.

The system can be run in windows, web server, Mac OS X, openBSD, freeBSD and platforms which support most Apache and IIS versions. The language is PHP version 4.3.3+. A database such as MySQL or PostgreSQL to store content and settings is required. Drupal has a blog system, a forum and a menu system. It can create either a classic or an interactive website.

Modules included in Drupal's 5.x core enable users to:
Post, revise, and categorize their content
Conduct a search on the internal site's content
Post their comments
Contribute in forums
produce collaborative writing projects
Add and view personal profiles
Communicate amongst themselves and with managers of sites
Change the look of website's using off-the-shelf and custom-made themes
Produce multi level menus
Provide an interface in their local language
Provide RSS feeds
Aggregate the RSS feeds of other website's
Register and manage user accounts
Assign user roles, allowing users permission to use certain selected features of the site
Use access rules to deny site access to specified users, email addresses and IP addresses
Provide statistics and reports for the administrators plus much more.......

Drupal core has been designed with a system of hooks or callbacks, that will allow modules to insert functions into Drupal's path of execution.
The functions and the presentation can be added through pluggable modules and themes.