System Overview
The TYPO3 system is multi-layered. The backend and frontend user interfaces sit on top of the application layer, which in turn sits on top of the infrastructure layer. The webserver, database and PHP in the infrastructure layer are prerequisites for running TYPO3.
The TYPO3 Core primarily consists of an API (Application Programming Interface), which defines a framework for managing content in the project. The API includes functionality for content storage, user permissions and access, content editing, and file management. This functionality is delivered via system extensions that use the API. All of the content is stored in a database that TYPO3 accesses via the API.
Extensions are clearly defined parcels of code, such as plugins, backend modules, application logic, skins, and third-party apps.
The most important thing to note is that the TYPO3 CMS consists solely of extensions. The most basic functions are packaged in a system extension called "core".
Diagram showing the layers of the TYPO3 system
Application layer
The TYPO3 Core framework interacts with system and 3rd party extensions via the TYPO3 extension API.
The core and extensions interact with each other seamlessly and operate as a single, unified system.
User interface layer
The backend is the content-creation side. It is an administrative area where you can manage content and configuration based on the extensions that are installed.
The frontend is the content-delivery side. Typically a website, it is the meeting point for templates, CSS, content, and logic from extensions, delivering your project to the world. The frontend doesn't have to be a website, it can be a native mobile application, a web application built in a frontend framework, or an API to interface with other systems.