Drupal is an open-source web development platform for online content. It is a type of website built on a framework known as a content management system. A Content Management System is a combination of tools that are put together to help organize, maintain and enhance content which can be made of text, graphics, sounds, video, audio, etc. One of Drupal's greatest strengths comes from giving users the ability to securely edit their online content at any time without needing to know html.
With a dedicated CMS such as Drupal, it's strength comes from an approach which emphasizes flexibility through modularity, where modules are designed to work well with the core system and expand its capabilities. What this translates to is choice, which in turn leads to a high degree of customization.