A Web service is a service offered by an electronic device to another electronic device communicating with each other via the World Wide Web.

In a web service web technology such as the HTTP originally designed for human-to-machine communication is utilized for machine-to-machine communication more specifically for transferring machine readable file formats such as XML and JSON.

In practice the web service typically provides an object-oriented web based interface to a database server utilized for example by another web server or by a mobile application that provides a user interface to the end user.

https://en.wikipedia.org/wiki/Web_service