Introduction

TYPO3 CMS is entirely built around the concept of extensions. The Core itself is entirely comprised of extensions, called "system extensions". Some are required and will always be activated. Others can be activated or deactivated at will.

Many more extensions - developed by the community - are available in the TYPO3 Extension Repository (TER).

Yet more extensions are not officially published and are available straight from source code repositories like GitHub.

It is also possible to set up TYPO3 CMS using Composer. This opens the possibility of including any library published on Packagist.

TYPO3 can be extended in nearly any direction without losing backwards compatibility. The Extension API provides a powerful framework for easily adding, removing, installing and developing such extensions to TYPO3.

Types of extensions

"Extensions" is a general term in TYPO3 which covers many kinds of additions to TYPO3.

The extension type used to be specified in the file ext_emconf.php, but this has become obsolete. It is no longer possible to specify the type of an extension. However, there are some types by convention which follow loose standards or recommendations. Some of these types by convention are:

Sitepackage is a TYPO3 Extension that contains all relevant configuration for a Website, including the assets that make up the template (e.g. CSS, JavaScript, Fluid templating files, TypoScript etc.). The Sitepackage Tutorial covers this in depth.
Distributions are fully packaged TYPO3 CMS web installations, complete with files, templates, extensions, etc. Distributions are covered in their own chapter.

Extension components

An extension can consist of one or more of these components. They are not mutually exclusive: An extension can supply one or more plugins and also one or more modules. Additionally, an extension can provide functionality beyond the listed components.

Plugins which play a role on the website itself, e.g. a discussion board, guestbook, shop, etc. Therefore plugins are content elements, that can be placed on a page like a text element or an image.
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.
Symfony console commands provide functionality which can be executed on the command line (CLI). These commands are implemented by classes inheriting the Symfony \Symfony\Component\Console\Command\Command\Command class. More information is available in Console commands (CLI).

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 losing 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.

Notable system extensions

This section describes the main system extensions, their use and what main resources and libraries they contain. The system extensions are located in directory typo3/sysext.

Core: As its name implies, this extension is crucial to the working of TYPO3 CMS. It defines the main database tables (BE users, BE groups, pages and all the "sys_*" tables). It also contains the default global configuration (in typo3/sysext/core/Configuration/DefaultConfiguration.php). Last but not least, it delivers a huge number of base PHP classes, far too many to describe here.
backend: This system extension provides all that is necessary to run the TYPO3 CMS backend. This means quite a few PHP classes, a lot of controllers and Fluid templates.
frontend: This system extension contains all the tools for performing rendering in the frontend, i.e. the actual web site. It is mostly comprised of PHP classes, in particular those in typo3/sysext/frontend/Classes/ContentObject, which are used for rendering the various content objects (one class per object type, plus a number of base and utility classes).
Extbase: Extbase is an MVC framework, with the "View" part being actually the system extension "fluid". Not all of the TYPO3 CMS backend is written in Extbase, but some modules are.
Fluid: Fluid is a templating engine. It forms the "View" part of the MVC framework. The templating engine itself is provided as "fluid standalone" which can be used in other frameworks or as a standalone templating engine. This system extension provides a number of classes and many View Helpers (in typo3/sysext/fluid/Classes/ViewHelpers), which extend the basic templating features of standalone Fluid. Fluid can be used in conjunction with Extbase (where it is the default template engine), but also in non-extbase extensions.
install: This system extension is the package containing the TYPO3 CMS Install Tool.