Drupal is an Open Source Content Management System.
In summary, it offers out of the box functionality to create a website with all the functionality you'd expect, such as excellent content management, categorisation of content, contact forms, menu building, and user management.
In addition to the out of the box functionality, there are many downloadable modules to add extra functionality, and the number of modules freely available grows rapidly.
Drupal can be used as a method to build run of the mill websites with ease, but can also be used as a framework for more complex business led applications.
It's biggest advantage is versatility, the core of drupal allows great flexibility in how new functionality can be added, and existing functionality changed.
