Microsoft ASP.NET AJAX, a set of JavaScript libraries built into ASP.NET 3.5, is available as a separate download for ASP.NET 2.0. Being an integral part of ASP.NET 3.5 and the AJAX Controls Toolkit for ASP.NET 3.5, now ASP.NET AJAX client-and server-side libraries are more integrated with Visual Studio 2008. The client-side library allows you to implement client-level processing such as processing and validating information entered by the end user, refreshing a portion of the web page, and developing rich, interactive user interfaces. You can also efficiently integrate the client-side library components with the server-side ASP.NET controls library in asynchronous mode. The key technology driver of ASP.NET AJAX is scripting. In general, script-based web applications face several challenges due to different browser settings (e.g., JavaScript is not enabled by default) on PCs and mobile devices. As a result, scripting is often not always the best strategy for enterprises to use to develop secure and scalable RIAs.
ASP.NET AJAX supports limited features of RIAs, and does not support effective multimedia integration, managed code-behind integration, or metadata and information management. Microsoft ASP.NET AJAX is a widely accepted model for building RIAs, but it is very likely that, having Silverlight as an option, .NET developers will migrate ASP.NET AJAX applications to Silverlight RIAs. Visit www.asp.net/ajax/ if you want to learn more about Microsoft ASP.NET AJAX
