Commonly referred to as GCM, Google Cloud Messaging served as C2DM's successor, making improvements to authentication and delivery, new API endpoints and messaging parameters, and the removal of limitations on API send-rates and message sizes. It is a message that pops up on a mobile device. It is the delivery of information from a software application to a computing device without any request from the client or the user.
GCM, Google Cloud Messaging, replaced C2DM. It improved authentication and delivery methods, added new API endpoints and parameters for messaging, and removed restrictions on how often applications could send messages and the size of those messages. This is a notification that appears on a mobile device and automatically delivers information from an application to it without user or client interaction.