There are several distinct parts that make up the functionality of ASP.NET AJAX. The core ASP.NET AJAX release was originally shown as part of a CTP. A CTP is a Community Technology Preview released before the product is ready for final consumption in order to share progress and gather early feedback. It was then released in supported form as an add-on to version 2.0 of the .NET Framework. It was subsequently incorporated into the .NET Framework 3.5 update.
The ASP.NET team continues to explore new feature investments and provides tech previews to gather input and share the work being done. The APIs and features in the previews will evolve over time as the work progresses towards a fully supported release. Bertrand LeRoy has provided some insight in his AJAX Roadmap document available at www.codeplex.com/aspnet/Release/ProjectReleases.aspx?ReleaseId=14924.
The ASP.NET AJAX Toolkit is a set of shared source behaviors and control extenders that are central to writing richer Web applications. These elements build on top of the core ASP.NET AJAX platform pieces to provide the key pieces necessary to push Web development further. Many developers view the Toolkit as a fundamental part of ASP.NET AJAX although they continue to be distinct installs.