web api used in distributed or interoperable or in dotnet only?

You can use api to communicate with any other system. 

 

 

An application programming interface (API), is a computing interface that defines interactions between multiple software intermediaries. It defines the kinds of calls or requests that can be made, how to make them, the data formats that should be used, the conventions to follow, etc. It can also provide extension mechanisms so that users can extend existing functionality in various ways and to varying degrees.

you dint get the purpose of web api,

suppose you want to integrate the likes count functionality of Facebook in your mobile application which will tell how many people have liked the current page of your application in facebook, the same you want to implement in a web application ,  Now for facebook offering the same functionality with UI for two platforms is a waste of time so they developed api for that for example http://fb.Like?yourpageurl , now "like" is a service (self contained independent working functionality) which is exposed via an interface named as Like , now as it is for various application like desktop, mobile, web we call it application programing interface (api).

 

so API is just interface which exposes some functionalities not the UI just in terms of data , which can be consumed by any client like javascript app, java app, .net app anything.