The IsPostBack property of the page object may be used to check whether the page request is a postback or not. IsPostBack property is of the type Boolean.
A postback is the name for the page request that an ASP.NET page sends to itself when a Web control is activated, such as when users click a button. Most Web forms use the HTML form control, which allows users to submit data using HTTP Post requests.
In an asynchronous model, all the server side events occur, as they do in a synchronous model. The Microsoft AJAX Library also raises client side events. However when the page is rendered, asynchronous postback renders only the contents of the update panel, where as in a synchronous postback, the entire page is recreated and sent back to the browser.
When an action occurs (like button click), the page containing all the controls within the <FORM… > tag performs an HTTP POST, while having itself as the target URL. This is called Postback.
Whenever you send a request to the server it sends request to the same page. Cross_page posting means it will send request to the another page. we can achieve. this through postback url property
The UpdatePanel enables you to add AJAX functionality to existing ASP.NET applications. It can be used to update content in a page by using Partial-page rendering. By using Partial-page rendering, you can refresh only a selected part of the page instead of refreshing the whole page with a postback.
ASP.NET UpdatePanel controls enable you to build rich, client-centric Web applications. By using UpdatePanel controls, you can refresh selected parts of the page instead of refreshing the whole page with a postback. This is referred to as performing a partial-page update. An ASP.NET Web page that contains a ScriptManager control and one or more UpdatePanel controls can automatically participate in partial-page updates, without custom client script.