apps .. hooks .. install .. storage .. system autoload controllers helpers i18n lib models tmp ui vendor
The root directory contains index.php file which is the starting point of the application
If you create custom plugin, you will put it here. By default it contains just a index.html file to prevent directory indexing.
All cores files are located here.
This folder contains application controllers
Built in classes are located here
This folder contains application models
This folder contains application helpers / built in functions etc.