Difference between Callback and Postback in asp.net?

               As with a postback, a callback does not refresh the currently viewed page.If there were two drop down boxes, the second dependant on the value of the first, when a user selects a value of a the first, rather then refreshing the page, doing some server side calculations and returning a new whole page to the client, a callback can enable you to only go fetch the required data. View State is not updated with a callback.