Updated: September 16, 2014
As explained in Registration Management, applications that use Notification Hubs can choose to access hubs from both their mobile applications and their app backends.
To better serve these two access patterns, Notification Hubs provides two sets of APIs, one to be used from mobile apps to register for notifications, and one to be used from the app back-end to perform registration management and send notifications. In addition, Notification Hubs exposes a REST API layer which includes all the functionality exposed through the back-end API.
Device APIs are only used to register devices from mobile apps (for security reasons, they do not expose Send methods). They automate the management of Notification Hubs-related information in the device local storage, and they support registration for a single PNS (for example, the Windows Store device APIs only register Windows devices).
Notification Hubs currently provides the following device API sets: