Drupal is an open-source Content Management System (CMS), which is particularly suited to customization through use of blocks, themes and modules. Drupal's innovative use of taxonomy and its unique menu system allow for effective management of content as a site grows or evolves.
Driving the ongoing development of Drupal is the desire for a simple and powerful framework which developers can work within to create custom web applications without having to build the pieces that nearly all websites have as a foundation (user authentication, content creation interface, layout managment).
And unlike other similar CMSs, Drupal takes its role as a framework very seriously. Take a look at the database structure of any other CMS that has been around for a while, and you'll almost certainly find a labyrinth of tables with mysterious names and seemingly redundant purposes. Drupal's database design is clean and constantly being re-factored. Tables that have outlived their purpose are removed from the schema and the code. Table design is very generic, allowing different types of content and modules to share the same table space.
Drupal also has an actual API. It too is refactored on a regular basis as the web and user needs change. Drupal 4.7, for example, has a completely redesigned Forms API which now allows developers a much broader reach into the entire Drupal framework, allowing her to tweak parts of Drupal that were previously pretty much set in stone.
From a webmaster's point of view, Drupal may not have the most eye-candy or largest feature set when compared to other CMSs. However, there's virtually nothing that can't be built into Drupal, making it the most flexible CMS out there.
Go with Drupal, and you will have no regrets.