Drupal is a content management system (CMS) that is widely used, in active develpment, and also happens to be very well written. That's the "Druplicon", which looks remarkably like the guy who started it all.
A "content management system" is a computer program (or programs) that makes it easy for you to build and maintain your website. Drupal is not a program on your computer, but on the computer that serves your website. To use it, all you need is the same tool you use to view websites, i.e. a "web browser".
To learn more about Drupal: