Client side update - PHP / ASP - email inbox

In gmail, I see that the inbox is automatically updated when an email is arrives. How is it possible in PHP or ASP to handle such sort of updating client side website from the server? I guess in the brow开发者_开发知识库ser chat it is the same process. Thanks.

This is called AJAX (Asynchronous JavaScript and XML), in which client-side scripts do request without refreshing the page altogether, and then changes page parts according to the returned response.