There are various definition of Push Technology.....
Push technology, or server push, describes a style of Internet-based communication.
In reference to the Internet or other online services, the technology whereby information is sent directly to a user's computer.
The delivery of web based content to the users desktop without the need for the user to visit a site to download information.
An Internet multimedia technology in which the user downloads special push play-in software that allows a Web site to send the user information
Automatically delivered information to a viewer who specifies some requirements; Push technology Compiles information from several sources. It is contrasted with pull technology where the user actively, searches for information (eg, by using a search engine).
Push technology refers to a set of technologies whereby information is delivered from a central server to a client computer
Push technology, or server push, describes a style of Internet-based communication where the request for a given transaction is initiated by the publisher or central server. It is contrasted with pull technology, where the request for the transmission of information is initiated by the receiver or client.
General use of Push technology:
Push services are often based on information preferences expressed in advance. This is called a publish/subscribe model. A client might "subscribe" to various information "channels". Whenever new content is available on one of those channels, the server would push that information out to the user.
Synchronous conferencing and instant messaging are typical examples of push services. Chat messages and sometimes files are pushed to the user as soon as they are received by the messaging service. Both decentralised peer-to-peer programs (such as WASTE) and centralised programs (such as IRC or XMPP) allow pushing files, which means the sender initiates the data transfer rather than the recipient.
Email is also a push system: the SMTP protocol on which it is based is a push protocol (see Push e-mail). However, the last step —from mail server to desktop computer— typically uses a pull protocol like POP3 or IMAP. Modern e-mail clients make this step seem instantaneous by repeatedly polling the mail server, frequently checking it for new mail. The IMAP protocol includes the IDLE command, which allows the server to tell the client when new messages arrive. The original BlackBerry was the first popular example of push technology for email in a wireless context.
Some of Push Technologies are:
HTTP server push
HTTP server push (also known as HTTP streaming) is a mechanism for sending data from a web server to a web browser. HTTP server push can be achieved through several mechanisms.
Generally the web server does not terminate a connection after response data has been served to a client. The web server leaves the connection open such that if an event is received, it can immediately be sent to one or multiple clients. Otherwise the data would have to be queued until the client's next request is received. Most web servers offer this functionality via CGI
Java pushlet
A pushlet is a technique originally developed for Java web applications, though the same techniques can be employed on other web frameworks as well. In this technique, the server takes advantage of persistent HTTP connections and leaves the response perpetually "open" (i.e. it never terminates the response), effectively fooling the browser into continuing in "loading" mode after the initial page load would normally be complete. The server then periodically sends snippets of javascript to update the content of the page, thereby achieving push capability. By using this technique the client doesn't need Java applets or other plug-ins to keep an open connection to the server.