Introduction to Web API

HTTP is not just for serving up web pages. It is also a powerful platform for building APIs that expose services and data. ASP.NET Web API is a framework for building web APIs on top of the .NET Framework. In general, you don't need to know ASP.NET MVC to use Web API.

See this nice poster http://www.asp.net/media/4071077/aspnet-web-api-poster.pdf

Unlike Controllers which inherits Controller base class, Web API controllers inherits ApiController class. You should use Web API if you're creating an API which exposes data. Use Controllers if you're writing HTML based application. (Difference between ApiController and Controller)

Web API Controller actions

Web controller action can return - (See more at asp.net)

• void (returns 204 - No content)
• HttpResponseMessage : returns Http Response. 
• IHttpActionResult : introduced in Web API 2. Simplifies unit testing
• Other types : Web API uses media formatter

Web API vs WCF (MSDN and SO)

• Web API supports only HTTP. 
• WCF Supports multiple  transports (HTTP, TCP, UDP)
• Web API supports wide variety of media types (XML, JSON)
• WCF supports multiple encoding types (Text, MTOM, Binary)
• Web API supports HTTP, Web Sockets, SSL etc. 
• WCF supports building services using WS-* standards

Web API vs ServiceStack

Routing (web-api-routing-and-actions)
Web API routing is similar to MVC routing. The main difference is Web API uses HTTP method, not URI path to select actions.

Here's a good article from asp.net about creating a RESTful API

Resources

http://www.asp.net/web-api
http://stackoverflow.com/questions/tagged/asp.net-web-api