A Web Service (also Webservice) is defined by the W3C as "a software system designed to support interoperable machine-to-machine interaction over a network. It has an interface described in a machine-processable format (specifically WSDL). Other systems interact with the Web service in a manner prescribed by its description using SOAP-messages, typically conveyed using HTTP with an XML serialization in conjunction with other Web-related standards." Web services are frequently just Internet Application Programming Interfaces (API) that can be accessed over a network, such as the Internet, and executed on a remote system hosting the requested services.

Other approaches with nearly the same functionality as web services are Object Management Group's (OMG) Common Object Request Broker Architecture (CORBA), Microsoft's Distributed Component Object Model (DCOM) or Sun Microsystems's Java/Remote Method Invocation (RMI). In common usage the term refers to clients and servers that communicate over the Hypertext Transfer Protocol (HTTP) protocol used on the Web. Such services tend to fall into one of two camps: Big Web Services and RESTful Web Services.

"Big Web Services" use Extensible Markup Language (XML) messages that follow the Simple Object Access Protocol (SOAP) standard and have been popular with traditional enterprise. In such systems, there is often a machine-readable description of the operations offered by the service written in the Web Services Description Language (WSDL). The latter is not a requirement of a SOAP endpoint, but it is a prerequisite for automated client-side code generation in many Java and .NET SOAP frameworks (frameworks such as Spring, Apache Axis2 and Apache CXF being notable exceptions).

Some industry organizations, such as the WS-I, mandate both SOAP and WSDL in their definition of a Web service. More recently, REpresentational State Transfer (RESTful) Web services have been regaining popularity, particularly with Internet companies. By using the PUT, GET and DELETE HTTP methods, alongside POST, these are often better integrated with HTTP and web browsers than SOAP-based services.

They do not require XML messages or WSDL service-API definitions.

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