This section describes the REST and AJAX features of the server side CallbackHandler service handler and the ww.jquery.js library that can be used to call it (although it's not required - you can use jQuery.ajax or similar directly).

• Using the CallbackHandler Class for easy REST Callbacks
• Direct HTTP Calls
• Creating Modal Dialogs for Client and Server Operation
• Error Handling in Ajax Controls
• Control Naming and Referencing from Client Script code
• Loading the client script library
• Loading jQuery and jQuery UI from a Content Delivery Network
• JavaScript Script Management Components
• Using Json.Net as the Serializer for REST and AJAX Callbacks

If you're using Web Forms you can also look for the AjaxMethodCallback control which makes usage of this functionality even easier.

WebForms Ajax Functionality