Web Service Description Language (WSDL)is a W3C specification which defines XML grammar for describing Web Services.XML grammar describes details such as:-
1- Where we can find the Web Service (its URI)
2- What methods and properties that service supports
3- Data type support.
4- Supported protocols
In short its a bible of what the webservice can do.Clients can consume this WSDL and build proxy objects that clients use to communicate with the Web Services.