A Web service is defined by the W3C as "a software system planned to support interoperable machine-to-machine interaction over a network”. Web services are often just Web APIs that can be accessed over a network, such as the Internet and implemented on a remote system hosting the requested services.

The W3C term refers to customers and servers that communicate using XML messages that follow the SOAP standard. In such type of systems, there is often machine-readable description of the operations presented by the service written in the Web Services Description Language (WSDL). The latter is not a necessity of a SOAP endpoint, but it is a condition for automated client-side code generation in many Java and .NET SOAP frameworks.