TYPO3 v8 has reached its end-of-life March 31st, 2020 and is not maintained by the community anymore.
You can order Extended Long Term Support (ELTS) here: TYPO3 ELTS.
TYPO3 can be extended in nearly any direction without loosing backwards compatibility. The Extension API provides a powerful framework for easily adding, removing, installing and developing such extensions to TYPO3. This is in particular powered by the Extension Manager (EM) inside TYPO3 and the online TYPO3 Extension Repository (TER) found at typo3.org for easy sharing of extensions.
"Extensions" is a general term in TYPO3 which covers many kinds of additions to TYPO3. The main types are:
Plugins which play a role on the website itself, e.g. a discussion board, guestbook, shop, etc. It is normally enclosed in a PHP class and invoked through a USER or USER_INT cObject from TypoScript. A plugin is an extension in the frontend.
Modules are backend applications which have their own entry in the main menu. They require a backend login and work inside the framework of the backend. We might also call something a module if it exploits any connectivity of an existing module, that is if it simply adds itself to the function menu of existing modules. A module is an extension in the backend.
Services are libraries that provide a given service through a clearly defined API. A service may exist both in the frontend and the backend. Please refer to the TYPO3 Services Reference for more information about this type of extension.
Distributions are fully packaged TYPO3 CMS web installations, complete with files, templates, extensions, etc. Distributions are covered in their own chapter.
Extensions and the Core¶
Extensions are designed in a way so that extensions can supplement the core seamlessly. This means that a TYPO3 system will appear as "a whole" while actually being composed of the core application and a set of extensions providing various features. This philosophy allows TYPO3 to be developed by many individuals without loosing fine control since each developer will have a special area (typically a system extension) of responsibility which is effectively encapsulated.
So, at one end of the spectrum system extensions make up what is known as "TYPO3" to the outside world. At the other end, extensions can be entirely specific to a given project and contain only files and functionality related to a single implementation.