Force.com Streaming API Developer Guide
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Push Technology
Push technology, also called the publish/subscribe model, transfers information that is
initiated from a server to the client. This type of communication is the opposite of pull
technology in which a request for information is made from a client to the server.
The information sent by the server is typically specified in advance. When using Streaming API, you specify the information that the client receives by creating a PushTopic. The client then subscribes to the PushTopic channel and is notified of events that match the PushTopic criteria.
In push technology, the server pushes out information to the client after the client has subscribed to a channel of information. For the client to receive the information, the client must maintain a connection to the server. Streaming API uses the Bayeux protocol and CometD, so the client to server connection is maintained through long polling.