Web API is Application Program Interface is framework for building HTTP services that can be consumed by broad range of client including browser,mobiles,Iphone and tablets.It can be used with MVC and other types of web applications like asp.net webforms etc.

Web API is HTTP Service that is developed for Mobile Application or Other web application. Web API provide service. For example it is like login from google and facebook.

web api is a middleware we use this for communication bw server & client