There are three different downloads available for the new AJAX platform. The Core Beta download is the one fully supported by Microsoft and has complete integration with ASP.NET, as well as support for networking stack, component model, and extender base classes.
The second download is called the Value-Add CTP Beta download, which has higher-level features but won't necessarily be supported mainstream at first, but, according to Microsoft. will eventually be integrated into the core redistribution. The third download is the ASP.NET AJAX Control Toolkit, a collection of 28 free tools and controls that enhance the features built-in to ASP.NET/AJAX.
All three are available on Microsoft's AJAX site.