Great whitepaper by Shannon Kalisky that provides a technical overview of ArcGIS Enterprise, its software components, key concepts such as federation, and outlines 3 options for completing a migration from a standalone ArcGIS Server deployment to the standard configuration of ArcGIS Enterprise called the base ArcGIS Enterprise deployment. It includes an outline of the key benefits, high-level steps, and important considerations for each migration option.
http://www.esri.com/library/whitepapers/migrating-arcgis-server-to-arcgis-enterprise.pdf