Liferay Portal is a very flexible platform that allows creating a wide variety of portals and websites. It is the developer through custom applications and customizations who gives it the shape desired by the end users of the portal. Liferay provides several tools (Plugins SDK and Liferay IDE) to ease this task. It also provides the foundations and frameworks to either implement completely new applications (portlet plugins) or customize the core functionalities and applications provided with Liferay (hook plugins and ext plugins).
As the official Developer's Guide for Liferay, this document has offered a description of each of the tools and frameworks that you as a developer can use to build the bests portals out there. Of course, while this document is large, it is just the beginning, the more you learn, the more efficient you will be while developing and the more interesting applications and customizations you will create. Here are some suggestions to learn more after reading this guide:
Read the "Liferay in Action" book. This book, written by Rich Sezov, Liferay's Knowledge Manager, provides a very extensive step by step guide of Liferay's development technologies.
Use Liferay's Community Forums, not only to ask questions but also to to answer them. You will be surprised how much you can learn while trying to help others.
Read the source. Liferay is Open Source, and you can leverage that to learn as much as you want about it. Download the code if you haven't done it yet and read it. Link it within your IDE so that you can enter Liferay's code while debugging your own code. It will give you a great opportunity to learn as much as the greatest expert of Liferay in the world.