This document discusses Google Cloud Messaging (GCM) in Android. It explains that GCM is used to send messages from the cloud to Android devices and applications to enable push notifications. It describes how C2DM, GCM's predecessor, worked and the limitations it had. GCM overcomes these issues and provides advantages like allowing developers to leverage cloud computing and build more productive apps. The document outlines how GCM works, involving communication between an app server, the GCM server, and Android devices. It provides examples of how GCM is used in Google Play to deliver app updates and downloads.