Google Cloud Messaging (GCM) for Android allows the communication between your server and your application using asynchronous messages. You need the client implementation in your Android app, but you also need the implementation in your server to send the messages and to store the client's data. You don't have to worry about handling low-level aspects of this communication, such as queuing and message construction. Using this service, you can easily implement a notification system for your application.
The following image shows how GCM works. The process involves three elements: your app, your server, and the GCM servers.
Your app first needs to register in GCM to get a registration ID, which identifies your app in a specific device.
Your app has to send the registration ID to your server so that it can be saved. This second step is necessary since your server needs to know the registration IDs of the user's device to send a message to your app in that device.
When your...