Drupal is a free, open source content management system (CMS) for web sites. It is distributed under the GPL license. While Drupal was first developed to build community websites, it has grown into a very powerful and flexible system that can be used for community websites as well as for enterprise websites or other Internet platforms.
The power of Drupal lies in it's very good documentation, flexibility and modularity which allows to build very complex and highly customized platforms. Its clear and modular structure make it easy for a developer to extend the existing functionality.
Drupal is a completely community driven project. There is a huge community developing, testing and using Drupal. Everyone can become part of the community and contribute to the project, be it as developer, by helping other users in the forums, writing documentation or reporting bugs found when using the product.
It is exactly this community of 10 thousands of contributors and millions of users which made Drupal one of the most powerful tools when it comes to websites. It is a very good example of the power of open source communities.
It would have been a huge challenge and investment for a single company to develop a system like Drupal. Ohloh estimates an effort of 19 person years and costs of more than 1'000'000$ for the development of Drupal core. While this is a lot, one has to know that due to Drupals modularity many of the important functionality is in the optional modules and not in core. Counting the huge pool of available modules the above estimations can easily be multiplied by 10.
Drupal may not be the best system to get a ready to use website right after installing it. Joomla could be better suited to do that. But for people with some development knowledge Drupal offers almost unlimited possibilities. It can be changed and extended to any degree. Thanks to its modularity this can be done without altering the core of the system but by simply adding modules, functions and styles. As such Drupal can be seen more as a framework than as a ready to go web content management system which has its limitations.
It is almost impossible to find a question answered with "no, this is not possible" in the Drupal forums. Everything is possible! The questions is just if Drupal can do it out of core, if one has to use some specific modules or if some additional code has to be written.
While "home users" may be better of setting up their website with Joomla, Drupal is definetly the tool for developers and web agencies. With Drupal it's possible to offer clients innovative, custom and stable solutions at lower costs.