Suppose you have three sets of data: Customers, Orders, Products.
You want to present three dropdownlists--one for each set of data. When I select a Customer from list A, list is rebuilt with Orders for that customer, the first order is selected, and list C is updated with the Products for that order, the first Product selected.
Using ASP.NET postbacks is pretty straightforward, but it's also pretty ugly. Al Alberto got us most of the way to a solution with this article on CodeProject. I've taken his work and extended to achieve the solution descibed above.