A TYPO3 extension is a module with special functionalities, which can be integrated into the TYPO3 basis system via defined interfaces.
A multitude of extensions for nearly all areas of application are available for TYPO3. They are split into categories (core, frontend, backend etc.). With use of the so-called Extension Manager these can be comfortably searched for, installed and configured within the TYPO3 backend.
Just like TYPO3, extensions are written in the programming language PHP. An extensive API (Application Programming Interface) offers ready-made categories and methods for the processing of data of different system areas.
The so-called shy extensions are nothing other than extensions. These are kept hidden by default within the Extension Manager to keep the extension lists more simple.
When creating your own extensions it makes sense to use the Extension Kickstarter.
Extensions can be installed globally or locally:
Global means that the extension is created under the
/typo3/ext/ folder of the TYPO3 source code and as a result it is available for all websites which use this TYPO3 installation.
Local means that the extension is saved under the
/typo3conf/ext/ directory. Due to the fact that this directory is located outside TYPO3 sources, it means that the extension is only available within the relevant installation.
Where possible we recommend installing extensions locally. This not only keeps the TYPO3 source code free of user-specific data but a further advantage is that local extensions are preserved during TYPO3 upgrades. This is not the case with global extensions.