Web API

WEB API :
WEB API :
A web API is an application programming interface (API) for either a web server or a web browser.

SERVER SIDE :
A server-side web API is a programmatic interface to a defined request-response message system, typically expressed in JSON or XML, which is exposed via the web—most commonly by means of an HTTP-based web server. Mashups are web applications which combine the use of multiple such web APIs. 
Webhooks are server-side web APIs that take as input a URI that is designed to be used like a remote named pipe or a type of callback such that the server acts as a client to dereference the provided URI and trigger an event on another server which handles this event thus providing a type of peer-to-peer IPC.
While "web API" in this context is sometimes considered a synonym for web service, Web 2.0 web applications have moved away from SOAP-based web services towards more cohesive collections of RESTful web resources. These RESTful web APIs are accessible via standard HTTP methods by a variety of HTTP clients including browsers and mobile devices.

CLIENT SIDE :
A client-side web API is a programmatic interface to extend functionality within a web browser or other HTTP client. Originally these were most commonly in the form of nativeplug-in architectures however most newer ones target standardized JavaScript bindings.
The Mozilla Foundation created their WebAPI specification which is designed to help replace native mobile applications with HTML5 applications.
Google created their Native Client architecture which is designed to help replace insecure native plug-ins with secure native sandboxed extensions and applications. They have also made this portable by employing a modified LLVM AOT compiler.