[TASK] Improve Extension concept definitions (#545)

* [TASK] Improve Extension concept definitions

Releases: main, 13.4

* Apply suggestions from code review

Co-authored-by: Garvin Hicking <[email protected]>

* Update Documentation/Concepts/Extensions/Index.rst

* Update Documentation/Concepts/Extensions/Index.rst

Co-authored-by: Mathias Bolt Lesniak <[email protected]>

---------

Co-authored-by: Garvin Hicking <[email protected]>
Co-authored-by: Mathias Bolt Lesniak <[email protected]>

Author: 3 people

Documentation/Concepts/Extensions/Index.rst

@@ -7,9 +7,92 @@
 Extensions
 ==========
 
-.. figure:: /Images/Illustrations/extensions.png
-
 Extensions are pieces of software developed by the TYPO3 community that extend
-the functionality of a TYPO3 installation. Extensions come in many forms
-- from small extensions that carry out specific tasks to larger extensions that
-provide an entire suite of functionality, e.g. the TYPO3 Blog Extension.
+the functionality of a TYPO3 installation. Extensions come in many forms.
+Some are only used for one site and contain mainly the theme of that site.
+These extensions are called a :ref:`site package <creating-a-site-package>`.
+
+..  contents::
+
+..  _concepts-extensions-composer:
+
+Extensions as Composer packages
+================================
+
+If you have worked with other PHP based projects you have probably run across
+`Composer packages <https://dev.to/joemoses33/create-a-composer-package-how-to-29kn>`__.
+
+Each TYPO3 extension is a Composer package of type `typo3-cms-extension`.
+Extensions provided by the TYPO3 Core have type `typo3-cms-framework`.
+
+The minimum needed to define a TYPO3 extension is:
+
+A directory containing a file called :file:`composer.json` with at least
+the following data:
+
+..  code-block:: yaml
+    :caption: packages/my_extension/composer.json
+
+    {
+        "name": "myvendor/my-extension",
+        "type": "typo3-cms-extension",
+        "require": {
+            "typo3/cms-core": "^13.4",
+        },
+        "extra": {
+            "typo3/cms": {
+                "extension-key": "my_extension"
+            }
+        }
+    }
+
+In order to be used the Extension should be
+:ref:`installed via Composer <installing-extensions>`.
+
+There is a legacy way to
+:ref:`install extensions without Composer <t3coreapi:extensions-legacy-management>` but it
+is not recommended anymore and not covered in this Guide. For this legacy way of
+installation as well as for functional tests or to publish your extension your
+need a file called :ref:`ext_emconf.php <t3coreapi:ext_emconf-php>`. This topic
+is also not covered here.
+
+..  _concepts-extensions-plugin:
+
+Extension vs plugin
+===================
+
+A TYPO3 extension is a similar concept to what is called a "Plugin" in WordPress.
+
+In TYPO3 a **plugin** is a **content element** that can be inserted into one or
+all pages, typically providing dynamic or interactive functionality.
+
+The data to be displayed is usually supplied by a special PHP class called a
+"controller".
+
+One **TYPO3 extension** can provide several plugins - or none at all.
+
+See also: :ref:`Plugins in TYPO3 (TYPO3 explained) <t3coreapi:plugins>`.
+
+Therefore in TYPO3 extensions and plugins are different concepts.
+
+..  _concepts-extensions-types:
+
+Types of extensions
+===================
+
+Internally the TYPO3 Core consists of mandatory and optional **system extensions**
+each of them is a Composer package. All mandatory system extensions and a few
+recommended ones will be automatically installed during the
+:ref:`Installation <t3start:installation-ddev-tutorial>`. Optional system
+extensions can be installed via Composer or the Extension Manager in classic mode
+
+**Third party extensions** offer additional functionality. Find commonly used
+extensions in the list of
+:ref:`Recommended Extensions <t3start:recommended-extensions>`. There are
+extensions available for many different use cases, see also chapter
+:ref:`How to find extensions <t3start:how-to-find-extensions>`.
+
+A **site package** is an extension that you install locally and only in your
+project. It contains the templates and assets as well as configuration for your
+theme. It can also contain specialized plugins or other pieces of software used
+only in this one project.