Web Services are client and server applications that communicate over the World Wide Web's (WWW) Hypertext Transfer Protocol (HTTP). As described by the World Wide Web Consortium (W3C), web services provide a standard means of interoperating between software applications running on a variety of platforms and frameworks.
Web Services can be looked as a code on demand. Just like we call functions and methods, web services can be looked upon as calling a function or method over the internet using some sort of protocols and some agreements.
A web service is a function or method which we can call by sending an HTTP request to a URL, with arguments and the service returns the result back as response.