Most eCommerce shops on the Internet require you to choose your country and state
when checking out to provide you with shipping costs. Instead of reloading the page
to get a list of states after you choose your country, we can use AJAX to do the
same thing -- minus the page refresh.
CascadingDropDown enables a common scenario in which the contents of one list depends
on the selection of another list and does so without having to embed the entire
data set in the page or transfer it to the client at all.