Atlas is a set of extensions for ASP.net provided by Microsoft that offers Ajax functionality, and has been described by some as “Ajax in Microsoft clothing”. Essentially, Atlas provides the same type of development platform for client-based web pages that ASP.net offers for server-based pages. However, it is not restricted to use with ASP.net. Ajax-style website development enables small packets of data to be exchanged asynchronously, so that users do not have to wait for a whole page to reload. Atlas in particular also provides a consistent set of application program interfaces for coding in Javascript. Atlas comprises client-side and server-side components. It integrates client script libraries with the ASP.net 2.0 server-based development framework, and is fully integrated with server-based services. Atlas also includes a network stack to simplify server connectivity and access to web services. For more information see Hot skills: Take on the Java world as Atlas battles with Ajax
How difficult is it to master?
Atlas makes it easier to work with Javascript for those not familiar with it, and .net developers should have no trouble adapting to Atlas.