Google Cloud Messaging (GCM) for Android allows communication between your server and your application using asynchronous messages. 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.
You have two options when using GCM:
The server can inform your app when there is new data available to be fetched from the server, and then the application gets this data.
The server can send the data directly in a message. The message payload can be up to 4 KB. This allows your application to access the data at once and act accordingly.
In order to send or receive messages, you will need to get a registration ID. This registration ID identifies the combination of the device and the application. To allow your app to use the GCM service, you need to add the following line to the manifest file of your project:
<uses-permission android...