The Drupal security team released security fixes and some bug fixes for Drupal, version 6.x and 5.x.
One of the security release is critical and it is recommended you upgrade ASAP.
The upgrade instructions can be found here.
Drupal is a very powerful open-source content management system built in PHP/MySQL/PostgreSQL (other database are supported too).
It runs best in an Unix/Linux/Apache environment and it has implemented a caching mecahnism which makes it quite scalable.
It has lots of features including multilanguage support, templating, caching, blogging, administration, community features and it is used by sites like Warner Bros, The New York Observer, Fast Company, Amnesty International, SpreadFirefox.
Many hosting companies offers this software as an easy instalable package so you can start right away. Drupal site is also rich in informations and tutorials. It has an active forum too where you can find support and even developers if you need them, not to mention books written about it (Amazon.com is a good place to start looking for those).
Another open source content management system which is a strong contender to Drupal and is widely used too is Joomla.
We will review Drupal soon and give you our opinions.