Web API (application programming interface) has two main interpretations. It is used to refer to both a server-side API upon a web server as well as client-side API within a web browser.
As per Microsoft, ASP.NET Web API is a framework that makes it easy to build HTTP services that reach a broad range of clients, including browsers and mobile devices. ASP.NET Web API is an ideal platform for building RESTful applications on the .NET Framework.
You can find tutorials here.
You can download HTTP Life Cycle here.
