Extensions add specialized functionality to the lean, mean Symphony core.
Extensions are add-ons that can provide additional functionality for your Symphony project. Some more common uses for extensions include providing additional [field types][fields] and text formatters, enhancing the admin interface, and even bundling entirely new features.
Extensions must be placed in the extensions/ folder of a Symphony installation in order to be made available to the system.
They can be managed in the admin interface, under System > Extensions. Individual or bulk management is possible by selecting extensions and using the "With Selected" dropdown to Enable, Disable, or Uninstall.
For more information, see Managing Extensions.
There are eight default extensions. These are bundled with the Symphony CMS download archive. If you clone Symphony from its Git repo, you will need to checkout the bundle branch and initialize the extension submodules therein.
The best places to find extensions, and to check for updates, are SymphonyExtensions.com and the extension download section of the Symphony website.