Explain what a SOAP web service is and how it is implemented using WSDL.

SOAP (Simple Object Access Protocol) is a protocol for exchanging structured messages in distributed computing, based on XML. The SOAP web service provides an interface for remote method invocation via HTTP or other protocols.

WSDL (Web Services Description Language) is an XML document that describes the web service interface: available methods, their parameters, data types, and addresses for invocation.

Implementing a SOAP web service using WSDL involves:

1. Creating a WSDL file that describes the service, operations, and messages.
2. Generating server and client code based on the WSDL (many tools do this automatically).
3. Developing server logic that handles SOAP requests.

Thus, WSDL serves as a contract between the client and the server, allowing the client to understand how to correctly invoke web service methods.