Disclaimer: This comparison is still WIP and further details for every solution will follow.

A push notification server is provides a server-side aggregation of different push notification server providers (like Google’s GCM, Apple’s APNs or Windows’ WNS). It helps you to reduce the complexity and maintenance effort of using a bunch of libraries to support each of those push notification services.

Some push notification servers even support the developer the manage user / client-subscriptions (even across platforms). Usually your backend communicates with the notification server through REST APIs. To sum up:

The basic functionality of a push notification server is to act as a gateway between your traditional backend and all the push notification service providers.

AeroGear UnifiedPush Server	Universal Mobile Push Daemon (pushd)	PushSharp	Uniqush	AirNotifier

GCM APNs MPS	GCM APNs WNS MPS	GCM APNs WNS	GCM APNs ADM	GCM APNs WNS/MPNS SMS
Java	CoffeeScript	C#	Go Redis	Python MongoDB Tornado