Joomla (more accurately "Joomla!" with an excalamation mark) is a content management system not unlike Drupal or Plone. It has many plugins and themes (known as templates).
Joomla has a worse reputation than Drupal. Some of its limitations:
- Has poor support for maintaining several sites from one installation.
- Installing plug-ins (or at least some) requires turning off the PHP safe_mode.
- Support for user-friendly/search-engine-optimisation-friendly URLs is worse.