Magento is an Open Source eCommerce platform developed using the Zend Framework by Varien. It is a fabulously customizable and extendible platform that once mastered can allow you to develop feature rich eCommerce solutions for your clients quickly and effectively. However for some mastering the Magento platform may not be that easy.
Magento uses common design patterns such as MVC, Application Controller and Two Step View to structure the application and provide mechanisms to extend and override the functionality and presentation. The platform has a huge number of files and directories that can cause confusion for newcomers to the platform. However this extended folder structure clearly defines where the platforms core files exist and where you can place your custom code and modules. Allowing you to extend the platform without modifying the core and breaking the upgrade process.
However daunting the platform appears or skeptical you may be due to the unfamiliar concepts, invest a small amount of time with the platform and it will soon start to payoff. This list of tutorials and resources will help you get up to speed quickly.