A Joomlaextension is a small collection of code files that you can add to your Joomla CMS to get extra features. Extensions are optional -- Joomla doesn't need them to run. You add an extension when you need a feature that Joomla doesn't already have.
In Joomla, "extension" is a general term that includes various kinds of things you can add to your web site:
- language packs
You can browse and download extensions at the Joomla! Extensions Directory.
Note that Joomla uses the words plugin and module differently from how WordPress and Drupal do. For more on this, and to understand the differences between Joomla components, plugins, and modules, visit the CMS Term Table.