The Need for Semantics

A Web Service Description defines the interface between the Client application and the Web Service.

WSDL only defines syntactic-level interface
Client and Service must also agree on semantics

"Semantics" = "meaning"
Can be oral or written (preferably)
Can be human-oriented (e.g., English) or machine-processable (e.g., RDF)
Should be referenced from WSD (use targetNamespace attribute)