TYPO3 is a free and open-source content management system (CMS) written in PHP. It is widely used for building and managing websites, web applications, and intranets. TYPO3 was first released in 1998 and has since evolved into a powerful and flexible platform with a large community of developers and users.

Key features of TYPO3 include:

1. Flexibility: TYPO3 is highly customizable and can be adapted to various requirements, from simple websites to complex web applications.

2. Extensibility: The CMS supports a wide range of extensions, allowing users to add new functionality easily.

3. Multilingual Support: TYPO3 provides robust multilingual capabilities, making it suitable for creating websites with content in multiple languages.

4. User Management: TYPO3 offers a sophisticated user and group management system, allowing administrators to control access levels and permissions.

5. Scalability: The CMS can handle large and complex websites, making it suitable for enterprises and organizations with extensive online needs.

6. Template Engine: TYPO3 uses TypoScript, a template language that enables developers to create dynamic templates and layouts.

7. Backend Interface: TYPO3 provides an intuitive and user-friendly backend interface for content editors and administrators to manage the website's content easily.

8. Versioning and Undo: TYPO3 includes versioning functionality, allowing users to track changes and revert to previous versions if needed.

9. Community and Support: The TYPO3 community is active and provides ongoing support, documentation, and regular updates to the CMS.

TYPO3 is a more advanced CMS compared to some other popular solutions like WordPress and Joomla. It is often favored by larger organizations or projects that require more complex features, such as multinational corporations, universities, and government institutions. However, it may have a steeper learning curve for beginners due to its extensive capabilities and configuration options.