This article provides GIS managers and analysts with an understanding of the fundamental concepts of Portal for ArcGIS and its capabilities. It presents a general overview of Portal for ArcGIS; discusses its key features, architecture, and implementation; and highlights the ArcGIS platform client applications that are available with Portal for ArcGIS.
Belated thanks for the kind feedback Jacob!
a must-read document for people trying to understand the general concept of Portal before they dive into more specific topics.
Here's another article you might find useful.
It describes the various levels of usage, integration, etc... for Server and it's various components in the WebGIS world (including Portal, Geo Processor.)
It's a good complement to Derek's paper, especially in terms of initial understanding and how the "new" WebGIS fits together:
ArcGIS 10.3.1 for Server Functionality Matrix
That's really nice, thanks for sharing Paul.