The architecture
Pimcore follows the MVC pattern, if you don't know what that means/is please read this article first http://en.wikipedia.org/wiki/Model%E2%80%93view%E2%80%93controller.
Since Pimcore depends on Zend Framework it also uses the MVC implementation of ZF.
If you are new to the Zend Framework or the the ZF MVC, please also read http://framework.zend.com/manual/en/zend.controller.html before you go to the next steps.
Basics
First of all you have to know a few basics (you also can find them in the editor guide).
Pimcore has 3 core modules:
Documents
This is the classic application of a web content management system (WCMS). You create templates, make them editable and create pages (fill them with content) which can be viewed under the specified address.
Assets
Manage static resources such as images, documents (pdf, docx, …), videos, … They can be inserted into the pages/snippets.
Objects (Classes)
The most powerful module. This can be used for RAD (Rapid Application Development). You can easily create a data model with drag & drop, the system creates the model for you in the backend (yes also the php files to work with).
Explanation of the files/folders
After the installation there should be 3 folders of Pimcore: /pimcore /plugins /website.
The only really interesting one is /website, this folder includes all things concerning your website. 
The website folder 