In the very first episode of Speak Easy Drupal, the LevelTen staff compare and contrast a variety of content management systems. While the focus of this podcast is comparing Drupal, Wordpress and Joomla, attention is also given to open source vs. proprietary systems. You'll learn the differences between each CMS, strengths, differences, and when you should use each.
The simple answer is that there isn't any. Each CMS has it's clear strengths and weaknesses. If you're looking for a quick answer, here's the lowdown:
A Content Management System or CMS, is a piece of software, such as Wordpress, Drupal, or Joomla, which allows an end user to build a powerful, dynamic website with little to no custom programming. Websites built with a content management system are typically easy to update and make changes to.