Directory structure

By default a TYPO3 installation consists of a structure of main directories within the web server document root. You will find this structure to be almost always like that. Depending on the installation variant you choose however, this may be slightly different. For instance, it is possible to have all PHP files except the entry points index.php within the composer managed vendor/ directory, outside of the document root. This setup however did not fully settle yet, and is not documented here in detail. So, if you look at "casual" TYPO3 installations, you will almost always find the directory structure as outlined below.

Directory	Description
fileadmin/	This is a directory in which editors store files. Typically images, PDFs or video files appear in this directory and/or its subdirectories. Note this is only the default editor's file storage. This directory is handled via the FAL API internally, there may be further storage locations configured outside of fileadmin/, even pointing to different servers or using 3rd party digital asset management systems. Note Note this directory is meant for editors! Integrators should not locate frontend website layout related files in here: Storing HTML templates, logos, Css and similar files used to build the website layout in here is considered bad practice. Integrators should locate and ship these files within a project specific extension.
typo3/	TYPO3 Backend directory. This directory contains most of the files coming with the TYPO3 Core. The files are arranged logically in the different system extensions in the sysext/ directory, according to the application area of the particular file. For example, the "frontend" extension amongst other things contains the "TypoScript library", the code for generating the Frontend website. In each system extension the PHP files are located in the folder Classes/. See extension files locations for more information on how single extensions are structured.
typo3conf/	TYPO3 configuration directory. This directory contains local extensions in typo3conf/ext folder. It may also contain a typo3conf/l10n directory that holds localisation files for frontend or backend languages other than english. The most important file within typo3conf/ however is LocalConfiguration.php. This one contains local settings of the main global PHP array $GLOBALS['TYPO3_CONF_VARS], crucial settings like database connect credentials are in here. The file is managed by the install tool and the extension manager and the content should not be managed manually since extension manager or install tool may override manually changed settings again. The file LocalConfiguration.php can be enriched by AdditionalConfiguration.php which is never touched by TYPO3 internal management tools. Be aware that having settings within AdditionalConfiguration.php may prevent the system from doing automatic upgrades and should be used with care and only if you know what you are doing.
typo3conf/ext/	Directory for local TYPO3 extensions. Each subdirectory contains one extension.
typo3conf/l10n	Directory for extension localisations. The "Language" module of the TYPO3 Backend manages this directory.
typo3temp/	Directory for temporary files. It contains subdirectories for temporary files of extensions and TYPO3 components.