WSDL, or Web Services Description Language, is an XML-based language used to describe the functionalities offered by a web service. It provides a standardized way to define the service's endpoints, operations, and the messages exchanged between the client and the server. This allows different systems to communicate effectively over the internet. A WSDL document typically includes information about the service's location, the methods available, and the data formats used. By using WSDL, developers can automatically generate code to interact with the web service, simplifying the integration process and ensuring compatibility between different software applications.