A Content Management System (CMS) is a software application, which will make it easy to create and maintain websites and manage all publishing formats associated with the World Wide Web. For more information, please have a look at the page Advantages of using a CMS.
TYPO3 is a such a CMS. It offers the best of both worlds: out-of-the-box operation with a complete set of standard modules and clean and sturdy high-performance architecture to accommodate virtually every kind of custom solution or extension. See Features TYPO3 CMS for a complete list of features.
TYPO3 is Open Source Software published under the GNU General Public License.