Joomla is a Content Management System that is General Public Licensed (GPL). General Public License means a license that is free and copyleft for software and other works. It provides you the right to change and share the versions of a program.
Joomla is a CMS in which it provides a structure through which you can add/edit content, change the look of a site etc. All of your information is stored in its MySQL database and it uses PHP script as a language. It does not require any knowledge of HTML or PHP. Web browser does all the work like publishing the content and configuration setting. It provides additional facilities or tools like online calendar, events, forums, guestbook etc.
A non-technical person can also manage the content of a website. Giving the burden of content management to a developer or to a technical person always is not fair.
HTML page where the content is to be published is called as a template which comes in the interest of a developer. The core engine of Joomla fetches the content from SQL database while run-time, merges it to template and publishes it as a resulting output web page.
Many open source and free templates are available to install for Joomla. It is beneficial for those who do not have high budget for a website; they can use Joomla to manage their website without worrying anything about budget, if they have the ability to install Joomla and have technical knowledge about operating Joomla.