cms
A website consists of content, design and navigation. A Content Management System (CMS) automatically deals with the latter two, so you can concentrate on the first. Additionally a CMS offers functionality like a search engine, web forms and version control.
A CMS consists of a database and software. Both run on the webserver. You can log in from any web browser on the CMS and edit your web content. A well configured CMS allows you to do this without any technical knowledge.
open source
Open Source software has some important advantages. As the source code is freely available, there is no vendor lock-in, you don't pay any license fees, and you can adjust or extend the software as you please.
A prerequisite for your choice of Open Source software is a healthy development community around the software project, so enough support options are available.
drupal
Drupal is an Open Source CMS. It is both powerful and flexible. Drupal's popularity is still growing. A strong community has gathered around the project.
Besides just a CMS, Drupal is a web application development platform. Its core is compact, and it is highly extensible and adjustable. A lot of modules are available, delivering additional functionality, and your own functions can easily be added.
A few well-known websites based on Drupal are MTV UK, Warner Brothers, Sony, Greenpeace UK and Ubuntu.