ASP.NET Ajax server controls consist of server and client code that integrate to produce rich client behavior. The following list describes the most frequently used Ajax server controls
ScriptManager Control UpdatePanel Control UpdateProgress Control
ScriptManager Control :
UpdatePanel Control :
UpdatePanel control is used to visually design the part of the page that is used in partial-page rendering. Placing controls inside an UpdatePanel control enables you to refresh selected parts of the webpage instead of refreshing the whole page with a postback to the webserver. When the UpdatePanel control performs an asynchronous post, it adds a custom HTTP header. There is no limit to the number of UpdatePanel controls that can be on a page, and they can be nested. Each UpdatePanel is updated individually and asynchronously, without affecting one another or anything else on the page.
UpdateProgress Control :
UpdateProgress control shows the status information for the progress of the download occurring in the UpdatePanel. The page can contain multiple UpdateProgress controls. Each one can be associated with a different UpdatePanel control. Alternatively, you can use one UpdateProgress control and associate it with all the UpdatePanel controls on the page.
Timer Control :
The Timer control enables you to perform postbacks at a specified interval. It can be used to trigger automatic updates to an area of the page that is wrapped with an UpdatePanel.