The Web Services Description Language is an XML-based interface definition language that is used for describing the functionality offered by a web service.
A WSDL definition is an XML document with a root definition element from the http://schemas.xmlsoap.org/wsdl/ namespace. The entire WSDL schema is available at http://schemas.xmlsoap.org/wsdl/ for your reference. The definitions element may contain several other elements including types, message, portType, binding, and service, all of which come from the http://schemas.xmlsoap.org/wsdl/ namespace.
The WSDL types element is a container for XML Schema type definitions. The type definitions you place here are referenced from higher-level message definitions in order to define the structural details of the message. Officially, WSDL 1.1 allows the use of any type definition language, although it strongly encourages the use of XML Schema and treats it as its intrinsic type system. The WS-I enforces this by mandating the use of XML Schema in the Basic Profile 1.0.