Core works on the following directory structure:

/
/class/
/class/extends/
/css/
/db/
/images/
/import/
/inc/
/lib/
/pages/
/templates/

all of the core structure is fairly self explanitory

core.class.php can auto load class extends via the global.php file.

core.class.php sits in the /class/ directory and global sits in /inc/