Push email is a type of e-mail system which delivers messages to an end user, even if he is not logged on. The push mail system is opposed to pull mail (traditional mail).
The pull mail service works like this. The mail user agent (mail application) checks the mail delivery agent (mail server) to see if new mail is available. If so, it downloads (pulls) the mail to the user mail box provided there is a network connection.
However, the user may not always be connected to the network. With the push mail system the server pushes the mail message to the user mail box, even if the user is not connected to the network.