Now featuring ASP.NET WEB APIs…
ASP.NET MVC was designed primarily for interacting with humans via web pages. The main use case is emitting HTML and responding to user input (submitting forms, clicking links, etc.). It does a great job at that.
ASP.NET Web API is built for all the other, non-human interactions your site or service needs to support. Think about jQuery code that's making an Ajax request, or a service interface that supports a mobile client. In these cases, the requests are coming from code and expect some kind of structured data and specific HTTP Status Codes.
These two are very complimentary, but different enough that trying to build out HTTP services using ASP.NET MVC took a lot of work to get right. The inclusion of ASP.NET Web API in ASP.NET MVC (and availability elsewhere, including ASP.NET Web Pages) means that you can build top-notch HTTP services in an ASP.NET MVC application, taking advantage of a common base and using the same underlying paradigms.
More at http://weblogs.asp.net/jgalloway/archive/2012/02/16/asp-net-4-beta-released.aspx
