Beginners/Understand How Joomla Works
From Joomla! Documentation
You might want to learn how Joomla! is built and how its features work together.
In short, Joomla! is composed of a Framework and extensions. There are several types of extensions, each capable of performing specific tasks. Some extensions are developed as part of the Joomla! project and come with your installation. If you want to add features to your site, you can install more extensions.
You can compare this to a computer operating system, like Microsoft Windows or Linux. You don't directly use the operating system (Joomla!); you use applications (extensions) to carry out tasks. Some applications (extensions) are immediately available on your system, but can install and remove additional applications (extensions) at any time.