A web API is just an application programming interface (API) over the web (that is, HTTP). When the resources of an application can be manipulated over HTTP using the standard HTTP methods of GET, POST, PUT, and DELETE, you can say that the application supports a web API for other applications to use.
http://www.hakimshabir.blogspot.com/2015/12/webapi-for-beginers-or-freshers.html
http://www.hakimshabir.blogspot.com/2015/12/webapi-for-beginers-or-freshers.html