Magento Local module file is relative to app/code/local/
Magento Community module file is relative to app/code/community/
Magento Core team module file is relative to app/code/core/
Magento User Interface (layouts, templates) is relative to app/design/
Magento Global Configuration is relative to app/etc/
Magento PHP Library file is relative to lib/
Magento Locale language file is relative to app/locale/
Magento Media library is relative to media/
Magento Theme Skin (Images, CSS, JS) is relative to skin/
Magento Other web accessible file is relative to /
Magento PHPUnit test is relative to tests/
Magento other is relative to /