Drupal is a content management system that can be used to manage a vast array of applications including but not limited to; membership websites, intranet and portals, social media sites and of course basic business websites.
Drupal is more than a CMS
In reality, Drupal is more than a CMS, rather it is a powerful framework which allows developers to work with modules that are completely customisable so rather than having a CMS where you have to build your site according to its restrictions, Drupal provides a CMS framework that is tailored and completely customised to your business.
Have you got different staff members who should only be able to edit certain sections of content whilst giving others complete control? No problem using Drupal your developer can set that up.
It's Open Source which means you are not locked in with a particular web developer, you own your site and you have a huge community behind you to support you. Drupal allows you the flexibility to build highly functional custom modules to accomplish near any task, there are many high-quality modules available for add-on capability.
Drupal is infinitely scaleable - want to build a social networking site, marketplace or directory website..no problem with Drupal.
Drupal is also well documented and is mature. It's huge community of supporters and developers mean you remain supported if you leave you current developer.