How Joomla! compares to an operating system
From Joomla! Documentation
If you're having trouble visualizing how Joomla! works, it might be useful for you to think of Joomla! as an operating system (OS), like Microsoft Windows, Apple Mac OS or Linux.
The most important similarity is that both your OS and Joomla! are not really intended for direct use. You need to install applications onto your OS (and extensions for Joomla!) to be able to actually do something useful with your computer (and your website). Just like your OS, Joomla! comes with a few core extensions that carry out the most important tasks.
Further similarities and differences can be found in the following table.
|Joomla! concept||OS concept|
|Core extensions||Built-in applications|
|File system||File system|
|Viewing a page||Using an application|
|Updating Joomla!||Installing updates for your OS|
|Migrating between Joomla! versions||Installing a new version of your OS|
(e.g. moving from MS Windows XP to MS Windows 7)