Drupal is a free, open source platform for building websites and software applications, developed by worldwide community and used by over a million websites, from personal blogs to some of the largest sites on the web.
Drupal is content management platform.
Drupal lets you easily publish and manage all kinds of content on your website : static and dynamic pages, images, podcasts, videos, calendars and events, polls, wikis, forums, etc. Fine-grained permission system gives you full control over different types of users and their access to content types on your site.
Drupal is framework.
Drupal provides flexible web development framework, able to accommodate almost any business, community or personal need. Built in PHP, it supports various server and database technologies, such as Linux and Apache, Windows and IIS, MySQL, PostgreSQL and more.
Drupal is community.
Drupal is build and supported by large and active community of people from all over the world, who collaborate not only online but on various community events - semi-annual Drupalcon conference and smaller events held around the globe.
Drupal is extensible.
Thousands of modules and design themes are available for Drupal, allowing you to build any kind of website. Just like Drupal itself, they are completely free. Drupal's extensible nature allows you to write custom modules/themes for any specific functionality/design you may need.
Drupal is secure.
Drupal has dedicated security team of professionals, who are constantly working on Drupal's code being up-to-date and protected against security threats.
Drupal is open source.
It is released under the GNU Public License and is completely free to download, use and modify. You don't need to pay any purchase, license or maintenance fees.