A web API is the application programming interface (API) for both the web server and web browser.
Web API is the great framework for exposing your data and service to different-different devices. Moreover Web API is open source an ideal platform for building REST-ful services over the Framework. Unlike WCF Rest service, it use the full features of HTTP (like URIs, request/response headers, caching, versioning, various content formats) and you don't need to define any extra configure settings for different devices unlike WCF Rest service.