TYPO3 is a free, open-source, enterprise-level content management system originally created by Kasper Skårhøj (a developer from Denmark) in 2000. It is a web-based application that we can run on a web server of our choice. The features that make up TYPO3 are modularized, separating all of the main sections and functions.
TYPO3 is released under the GNU (General Public License). This means that:
We are free to modify the source code of TYPO3 for whatever purpose we need.
We save money on software, and so we can focus on customization.
It is open source, so security vulnerabilities are more likely to be spotted and quickly patched.
The down side to the license is that the software comes with no warranty. Full details on the license can be found here: http://www.fsf.org/licensing/licenses/gpl.html.