A cascading dropdownlist is a group of two or more dependent DropDownList controls in which one DropDownList control depends on the parent or previous DropDownList controls. The items in the DropDownList control are populated based on an item that is selected by the user from parent DropDownList control. When the selection of a parent DropDownList control changes, the Web page calls the AJAX Web service to retrieve the list of items for the child DropDownList control.
The ASP.NET AJAX Toolkit provides a CascadingDropDown control as an ASP.NET AJAX extender. You can use CascadingDropDown control for creating a cascading drop-down list.
Country-State-City cascading dropdownlist. Here user want to select state on the selection change of Country and City dropdownlist on change of State. In this case State and City items are dependent to the Country, so you can use Cascading dropdownlist control.