<PLEASE NOTE Google Cloud Messaging has officially been shut down in May, 2016>
A service that allows you to send data from your server to your users' Android-powered device, and also to receive messages from devices on the same connection.
Versatile Messaging Targets - Distribute messages to your client app in any of three ways — to single devices, to groups of devices, or to devices subscribed to topics.
Downstream Messaging - For purposes such as alerting users, chat messaging or kicking off background processing before the user opens the client app, GCM provides a reliable and battery-efficient connection between your server and devices.
Upstream Messaging - Send acknowledgments, chats, and other messages from devices back to your server over GCM’s reliable and battery-efficient connection channel.