The Joomla project maintains a website called the Joomla Extensions Directory or JED ( http://extensions.joomla.org) that lists many of the extensions available from developers in the community.

From a programming point of view, the word extension has a slightly different meaning. In this sense of the word, an extension is any component, module, plugin,
language, or template. Extensions that are included with the standard Joomla download are called core extensions. Extensions from other sources, such as the JED, are
called third-party extensions.

A key point to understand is that, when Joomla is running, it doesn’t see any difference between core extensions and third- party extensions. That is why third-party
extensions appear to be completely integrated into the package. They are treated exactly like core extensions. If you are using a Joomla website that incorporates well-written third-party extensions, it can often be difficult to tell where the core package leaves off and an extension begins.