TYPO3 is an enterprise-level, open-source PHP Content Management System (CMS) known for its robust flexibility, scalability, and strong security, ideal for complex, large-scale websites, multilingual portals, and multi-site management from one installation.

Key characteristics include extensive customization via extensions, granular access rights, built-in features for SEO & social media, content staging, a responsive backend, and reliable long-term support, making it suitable for demanding digital platforms.

TYPO3 offers extensive customization via powerful templating (Fluid), configuration (TypoScript, TSConfig, TCA), and a flexible Extbase framework, enabling deep adaptation of backend interfaces (like the Form Editor's Inspector), frontend rendering, and custom logic through Dependency Injection, allowing developers to tailor everything from field visibility and form element behavior to complex application functionality, ensuring high flexibility for diverse project needs.

Once a page exists (in the page-tree), you assign it a Backend Layout. This tells TYPO3 how to arrange the editing area (e.g., a "Single Column" layout vs. a "Two Column" layout with a sidebar). These layouts are usually predefined by the site developer.

Within this structure you place predefined Content Elements (proposed by the TYPO3 core, extensions or the site developer) that handle the content very much seperated from grafics, style and programming.