Magento auto loader is the class Varien_Autoload
This class defines the php function spl_autoload_register() which is used to dynamically require classes.
When php system encounters any class which it not able to resolve, it looks at spl_autoload_register if it can resolve it.
Below is a gist of autoload function
$classFile = str_replace(' ', DIRECTORY_SEPARATOR, ucwords(str_replace('_', ' ', $class))); $classFile.= '.php'; return include $classFile;
so basically magento explodes class names by “_” and replaces it will “DIRECTORY_SEPARATOR” and the finally includes the files.