Core is the Joomla distribution file system "out of the box" (without third-party extensions) and basic CMS functionality.
The word "core" in relation to Joomla refers to the distribution files and basic functionality, which are also called "core extensions":
- managers for managing extensions and content,
- basic Joomla components (Contents, Contacts, Users, Search),
- Joomla modules (HTML code, Site Navigator, List of Contents, etc.),
- Joomla plugins (authentication, editors, editor buttons, etc.),
- Joomla templates.
FYI
Kernel extensions should not be confused with third-party extensions that are available for download from the JED.