Push Technology

Push technology, or server push, is a style of Internet-based communication where the request for a given transaction originates with the publisher or the server. It is in contrast with pull technology, where the request for the transmission of information originates with the receiver or the client.

General use
Push services are often based on information preferences expressed in advance. This is known as 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 decentralized peer-to-peer programs and centralized programs allow pushing files, this 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. 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 original BlackBerry was the first popular example of push technology in a wireless context.
Another popular type of Internet push technology was PointCast Network, which gained popularity in the 1990s. It delivered news and stock market data. Other uses are push enabled web applications including market data distribution (stock tickers), online chat/messaging systems (web chat), auctions, online betting and gaming, sport results, monitoring consoles and sensor network monitoring.

source : wikipedia