A web service is a software system designed to support interoperable machine-to-machine interaction over a network. It is a method of communication between two electronic devices over a network, where a client invokes a web service by sending an XML message, then waits for a corresponding XML response. Web services are self-contained, modular, distributed applications that can be described, published, located, or invoked over the network to create products, processes, and supply chains. They are built on top of open standards such as TCP/IP, HTTP, Java, HTML, and XML.

Web services can be offered by an electronic device to another electronic device, communicating with each other via the Internet, or a server running on a computer device, listening for requests at a particular port over a network. They are XML-based information exchange systems that use the Internet for direct application-to-application interaction. Web services are not tied to any one operating system or programming language, which means that software applications written in various programming languages and running on various platforms can use web services to exchange data over computer networks like the Internet in a manner similar to inter-process communication on a single computer.

In summary, a web service is a method of communication between two electronic devices over a network, where XML is used to encode all communications to a web service. It is a software system designed to support interoperable machine-to-machine interaction over a network. Web services are self-contained, modular, distributed applications that can be described, published, located, or invoked over the network to create products, processes, and supply chains. They are built on top of open standards such as TCP/IP, HTTP, Java, HTML, and XML.