As web 2.0 gets a major consideration among the web development technology, AJAX has taken the place here by providing rich user experience. All though it has not been possible to provide FULL strength of operating system UI, but AJAX gets popular due to it's new approach.
Although AJAX is providing a new, rich experience to the users, but the technology, based on which it is being utilized is not new at all. AJAX is basically the acronym of Asynchronous JavaScript and XML, as the elaboration says, it uses JavaScript and XML in asynchronous way.
For developers AJAX might be a bit tricky to learn JavaScript stuffs, especially those who are not habituated with client side coding. Microsoft made its developers get rid from this pain, by introducing a complete framework that wraps AJAX functionality into .net (.net 2.0 and VS 2005 however), which is Atlas. Currently it’s getting my concentration, although I have found that the latest release CTP July 2006 contains some bug. I am using this in my latest project and hoping that the bugs of Atlas will be resolved when I release my project as well as ;)