That's a big question. Drupal is an open source content management framework for powering websites large and small. For more information, see this community handbook page overview. You can download Drupal, read the documentation and join the worldwide Drupal community at Drupal.org.
Here's a one-minute answer:
We use Drupal because of its power and flexibility. Of all the content management systems, Drupal affords the most robust options for features and functionality to meet an almost infinite variety of use cases.
