ASP.NET AJAX is a set of technologies to add AJAX (Asynchronous JavaScript And XML) support to ASP.NET. It consists of a client-side script framework, server controls, and more.
The ASP.NET 2.0 AJAX Futures December CTP provides additional features and functionality that work alongside the ASP.NET AJAX Extensions. Together, these releases enable development scenarios supported in the earlier ASP.NET AJAX CTP releases in addition to newly added functionality.