eZ Publish 5.x is a "Dual kernel" software. This means it contains both = a legacy kernel (effectively a "4.y" version) aka "Legacy= Stack", and a new 5.x kernel, aka "Symfony Stack" (because it use= s Symfony2 Fullstack framework).
For documentation on the legacy kernel, please go to doc.ez.no w= here you'll find documentation on everything from its GUIs, official extens= ions to template coding ( 5.x doc: User man= ual & Technical manual ). If yo= u are looking for documentation on how to develop extensions for legacy in = php, then also see relevant material on share.ez.no ( Tutori= als, Blogs ).
The new 5.x kernel is built using an HMVC model. It currently consists o= f Public API (Model), Symfony2 Fullstack Framework (Hierarchical = View Controller) with Twig (template engine) enhanced with eZ Publish = concepts (Core Bundle). On top of these you'll find REST v2 API, Demo = Bundle and a Legacy Bundle allowing some integration between the two kernel= s.
For documentation on Symfony2 go to symfon= y.com (for Twig see here)&n= bsp;and for the eZ Publish parts of 5.x kernel continue reading in this sec= tion.