When any single request is made to Joomla it always loads and executes in the same pattern. Once you know how this pattern works you can make the right decisions on how to design your extensions.So all requests in Joomla either go to index.php at the root, or they go to the index.php file in the administrator folder.
In very simple PHP applications, many people will choose to execute their code in the order the results appear on the screen. On the other hand Joomla runs each module and components separately, then assembles everything into the template.
Joomla is designed in such a way that you never have to touch the core files. Aiding in this, Joomla provides five different types of extensions giving you a wide range of control over how Joomla loads and executes. It's possible to write all five types of extensions for both the site and administrator applications of Joomla
While most people know Joomla as a content management system, Joomla is also a platform where any number of applications can sit on top. Let's take a look at three that are prepackaged with the CMS. So first go to the root of a fresh copy of a Joomla installation and open up index.php.