WSDL is the Web Service Description Language, and it is 
implemented as a specific XML vocabulary. While it's very 
much more complex than what can be described here, there 
are two important aspects to WSDL with which you should be 
aware. 

First, WSDL provides instructions to consumers of Web 
Services to describe the layout and contents of the SOAP 
packets  the Web Service intends to issue. It's an 
interface description document, of sorts. And second, it 
isn't intended that you  read and interpret the WSDL. 

Rather, WSDL should be processed by machine, typically to 
generate proxy source code (.NET) or create dynamic proxies 
on the fly (the SOAP Toolkit or Web Service Behavior).

Web Service Description language
WSDL is an XML format for describing network services as a 
set of endpoints operating on messages containing either 
document-oriented or procedure-oriented information.