A free, feature rich, Content Management Framework built with PHP and running under many Unixes and Windows. The complete feature list is available at typo3.com.
If you were impressed by the feature list you're probably asking yourself if this product is for you? Be aware that all the flexibility and richness come with a price: complexity. If you're not ready to spend a month learning the system and are in a hurry to satisfy a customer, you should probably look into getting somebody to help you or look for something else.
TYPO3 is a very huge and capable system and it cannot be fully learned in a week! TYPO3 will always have a long learning curve for developers. Once mastered, the system will let you do the most complex website and the authors using it will love you for the choice of TYPO3.
People on all levels are working with TYPO3. However only people with internet technical skills will be able to install TYPO3 and develop websites with it (called "Developers"). The people with less knowledge ("Users") are typically the ones who paid someone else to do the technical job so they could maintain their website with the system.
-- Typo3.org