Assembly: System.Web (in system.web.dll)
Classes that implement the ICallbackContainer interface provide controls with a script that will initiate a callback to the server. A callback differs from the more common postback in that the page processing for the callback recreates the page without changing state information held by the page. For more information on creating callbacks, see Client Callbacks Without Postbacks in ASP.NET Pages.
Classes that implement the ICallbackContainer interface should also implement the ICallbackEventHandler interface to handle the events raised by controls.