PostBack is the name given to the process of submitting an ASP.NET page to the server for processing. PostBack is done if certain credentials of the page are to be checked against some sources (such as verification of username and password using database). This is something that a client machine is not able to accomplish and thus these details have to be 'posted back' to the server.
This property is automatically set to False whenever a page is called for first time and whenever a server redirect a new instance of a page this property is set to True.
This property is by default set to False. So if we perform some action on control such as DropDownList, no events are performed because events are not recognized.