The Joomla package consists of many different parts, which allow modular extensions and integrations to be made easily. An example of such are extensions called "Plugins". (Previously known as "Mambots".) Plugins are background extensions that extend Joomla with new functionality. The WikiBot, for example, allows the author of Joomla content to use "Wikitags" in Joomla articles which will auto-create dynamic hyperlinks to Wikipedia articles when displayed. There are over 3,500 extensions for Joomla available via the Extensions Directory.

In addition to Plugins, more comprehensive extensions are available. "Components" allow webmasters to perform such tasks as build a community by expanding user features, backup a website, translate content and create URLs that are more friendly to search engines. "Modules" perform such tasks as displaying a calendar or allowing custom code like Google AdSense etc., to be inserted within the base Joomla code.

Since it has been around longer, there are more extensions available for Joomla 1.0 than for Joomla 1.5, although native 1.5 extensions are becoming increasingly available. Some of the older 1.0 extensions can be used with version 1.5 if it is set to legacy mode.

Joomla permits administrators to set global configuration parameters that affect every article. Every page conforms to these parameters by default, but a page can have its own setting for each parameter. For example, you can elect to show or hide the article author or simply go with the global "show author" parameter.

Joomla has an official and many unofficial communities. By July 2008, the official Joomla forum had more than 300,000 threads and over 1.3 million posts from more than 200,000 registered members in 40 languages. Unofficial sites are published in many languages, often with Joomla extensions that are region specific. Bi-directional text support for the Hebrew and Arabic languages, for example, can be found on 3rd party community portals. Unofficial web developers also build extensions and web templates for commercial sale and offer freelance customization services. Usually a template is distributed as a zip file which can be installed using the Joomla installer.