PubNub is a product for developers to implement real time push notifications easy.
You might have been doing real time messaging for a long time and wonder what's with PubNub? Yup, if you don't use PubNub, you can still do real time messaging be using XMPP or jabber technology.
However, with PubNub, you can do away with setting up your XMPP server and worry about max connection per thread etc.. PubNub makes it easy to use for your server and client side.
Like all cloud service, this comes with a fee. It is free if you send 1 million messages per month (similar to UrbanAirship/Xtify). Otherwise, pay $15 per month and additional $1 for each 1 million messages.