What are all the directories for?

Question

Drupal 8 re-arranged all the directories and locations of modules/themes etc. Where do things go now?

Answer 1

• /core contains all the files provided by core that doesn't have an explicit reason to be in the / directory.
• /libraries contains third-party libraries. It's not used by Drupal core, but it's commonly found in many sites.
• /modules is the directory into which all custom and contributed modules go.
  This directory contains the contrib and custom sub-directories, which can be used to make easier to keep track of the modules.
• /profiles contains contributed and custom profiles.
• /themes contains contributed and custom (sub)themes.
• /sites is the directory used for running Drupal multi-site setups and it's used in the same way as in Drupal 7. Modules and themes in /sites/all/modules and /sites/all/themes will override the ones in the base modules and themes directories. Modules in /sites/SITENAME/modules and /sites/SITENAME/themes will override others for the given site.
• /vendor is the directory where Composer dependencies go, for example Symfony and Twig.

Details on the /core directory, primarily useful to know for new core hackers:

• /core/assets - Various external libraries used by Core. jQuery, underscore, modernizer etc.
• /core/misc - Frontend code that Drupal Core depends on.
• /core/includes - Functionality that is to low level to be modular. Such as the module system itself.
• /core/lib - Drupal Core classes.
• /core/modules - Drupal Core modules.
• /core/profiles - Drupal Core installation profiles. Minimal, Standard, Testing and Testing multilingual installation profiles by default.
• /core/scripts - Various CLI scripts, mostly used by developers.
• /core/tests - Drupal Core tests.
• /core/themes - Drupal Core themes.