PubNub's core focus is on getting data to millions of devices quickly, for example, Social TV companies run apps that synchronize content on iOS and Android mobile devices with live TV. PubNub works by broadcasting data: with PubNub, each phone subscribes to a channel while the server publishes data to that channel. Thus, millions of phones can subscribe to the same channel where PubNub replicates and delivers data to each device simultaneously (less than 1/4 second on average).
Apple Push Notification System (APNS) is designed to send data to one device at a time, with no guarantees on latency. APNS is ideal for waking up the app when the app is not being currently used (i.e. inactive, background, not running). When APNS and PubNub are used together, APNS is used for alerts to a user, and when the app is launched, it connects to a PubNub channel for very fast connectivity and data synchronization across the audience of people using the app.