Joomla! is a free open source Content Management System (CMS), it uses a "backend" system through which the "frontend" can be managed. This system is being used from small personal blogs to large corporate sites and intranets. The system includes features such as page caching to improve performance, RSS feeds, printable versions of pages, news flashes, blogs, polls, website searching, and language internationalization. Joomla is licensed under the GPL, and is since 2005 the result of a fork of Mambo.
From my experience with Joomla! I can say that it more complex and has a bigger learning curve then Drupal. However, Joomla! has a large community which provide many extensions (modules) and feedback. In case your needs are far beyond the basic CMS possibilities and you have enough time to spend (or the possibility to hire a professional), this is the CMS system you are looking for.
Drupal is an open source modular framework and Content Management System (CMS), it uses a "backend" system through which the "frontend" can be easily managed. This system is being used from small personal blogs to large corporate sites.
From my experience with Drupal I can say that it less complex and easier to manage then a CMS system like Joomla! On the other hand Joomla! offers a lot more possibilities. In case the Drupal features, available modules (extensions), themes and translations satisfy your business needs, it is an excellent choice. Otherwise you should have a look at Joomla!