Asp.Net Ajax provides ModalPopupExtender Control to create modal popup dialog. Basically the dialog is a panel control that associated with ModalPopupExtender. User have to click a button, hyperlink or controls to show this popup. We can make better dialog look and feel to the panel.
We need to add reference to AjaxControlToolkit.dll to build an ajax web page with all controls in AjaxControlToolkit. Also we have to register AjaxControlToolkit assembly, namespace and tagprefix.