/ app – is where the application resides
/ app / etc – is global configuration den
/ app / code – is where modules installed their models and controllers
/ app / code / core – are core team developed or certified modules
/ app / code / community – are community contributed modules
/ app / code / local – are local customizations
/ app / code / core / Mage – magento default namespace
/ app / code / core / Mage / {Module} – module root
/ app / code / core / Mage / {Module} / etc – module configuration
/ app / code / core / Mage / {Module} / controllers – controllers provided by module
/ app / code / core / Mage / {Module} / Block – Block logic classes
/ app / code / core / Mage / {Module} / Model – Object Models provided by module
/ app / code / core / Mage / {Module} / Model / Mysql4 – Resource Models provided by module
/ app / code / core / Mage / {Module} / sql – sql installation and upgrade files between module versions
/ app / code / core / Mage / {Module} / sql / {resource} / – resource model specific upgrades
/ app / code / core / Mage / {Module} / sql / {resource} / {type}-{action}-{versions}.(sql|php) – resource update files. example: mysql4-upgrade-0.6.23-0.6.25.sql
/ app / design – is location of design packages (layouts, templates, translations)
/ app / design / {package} / {theme} – theme customizations
/ app / design / {package} / {theme} / layout – .xml files that define block structure for different cases in website flow
/ app / design / {package} / {theme} / template – .phtml (html with php tags) templates
/ app / design / {package} / {theme} / locale – Zend_Translate compatible translation files for package/theme
/ app / locale – locale files
/ app / locale / {locale (en_US)} – Zend_Translate compatible translation files for modules
/ skin / {package} / {theme} / – is where design package css and images are
/ lib – are libraries such as Zend and Varien
/ js – are javascripts smile
/ media – uploaded files (product images, pdf documents, etc)
/ tests – Unit tests (not done yet)
/ var – temporary files
[tag]Magento folder[/tag]