Generally speaking, the architecture of this database-based Web Content Management Systems envisages that, from an Apache web server the TYPO3 Core will draw on the stored content of a MySQL (or another SQL-) database via commands in PHP. The TYPO3 Core consists mainly of a database model and its access functions in the Backend, a directory structure, user administration and access rights management. Any functions that go beyond these are managed as extensions, which are either plug-ins or modules. With the TYPO3 API extension, any number of further TYPO3 extensions can be connected to the core. Indeed, even the Frontend is programmed as an extension and can be exchanged for another if required, or even entirely set aside.
Thanks to this modular construction, the basic application – the TYPO3 Core – is kept strictly separate from those functions that do not form a necessary part of the usual "service package". By using the Extension Managers and the API extension, newly developed extensions can be integrated into the range of services of the TYPO3 CMS, from where they can then interact with the T3 Core Functions via the API (Application Programming Interface). This is what makes TYPO3 ideal as a pure development framework for online applications.
There is a wealth of examples of the way that Online Applications can be used: these go from an integrated Customer Self-Service Toolkit on a mobile telephone provider's website, through an Onboard Entertainment Service System on cruise liners, to using TYPO3 as an operating system for DVD players. In principle, anything is possible!