push notification explanation

What are push notifications and how do they work? Also wh开发者_如何学Pythonat is the difference with googles cloud to device messaging? Is c2dm also a push notification? Also do I need Special server setup to implement push notification?are push standard or every device has different implementation?

Basically the server maintains a list of connected clients. Whenever something of interest occurs, the server sends the data/notification/updated state to the connected clients. This is in contrast with pull, where the clients poll the server for changes.