The ASP.NET Web API is defined as a framework that allows us to build HTTP services for browsers and mobile devices and also it is an ideal platform for developing RESTful applications on the ASP .NET Framework.

 In asp.net web API is an extensible framework for building HTTP based services that can be accessed in different applications on different platforms such a web, windows, mobile devices, etc.

Why used Web API?

 Google Trends WCF vs. ASP.Net web API

The pros/benefits of using web API:

• The Web API provides supports for convention-based CRUD Actions since it works with HTTP verbs ( GET, PUT, POST, and DELETE)
• Web API is easy to define, expose, and consume in a Restful way. 
•  The ASP.Net Web API allows us to showcase our data and services to various devices.
•  The Web API is open source, so it is an ideal option for creating RESTful services over the .NET platform.
•  It allows us scalable, decoupled development of a Web Service which can be shared and used by other developers to access web services.
•  The Web API is also helpful to build simple, non-SOAP-based HTTP Services.

The Characteristics of Web API

•  Tightly coupled to HTTP
•  Easy to test
•  Offers support for content negotiation
•  Allows open-ended formatting
•  No reliance on a platform
•  Solid resource-orientation
•  Level 2 experience with nascent hypermedia support