Push (or "server-push") is the delivery of information on the Web that is initiated by the information server rather than by the information user or client , as it usually is. An early Web service that specialized in "pushing" information rather than having it "pulled" as the result of requests for Web pages was Pointcast, a site that provided up-to-date news and other information tailored to a previously defined user profile. Marimba was a somewhat similar site (and product) that pushed information to the user on a predefined schedule.
In fact, the information pushed from a server to a user actually comes as the result of a programmed request from the client in your computer. That is, any information pusher on the Web requires that you download a client program. This program captures your profile and then periodically initiates requests for information on your behalf from the server.
A truer form of push is broadcast information. In this case, the information is pushed to everyone that has access to a particular channel or frequency. Broadcast usually (but not always) involves a continuous flow of information.
Another form of "pushed" information is e-mail. Although the e-mail client in your computer has to occasionally go to your local e-mail server to "pick up" the e-mail, the e-mail arrived because someone sent it (pushed) it to you without a one-for-one request having been made.