What? is the interface

The interface is an abstract model of the service:

• references the set of messages that a service sends or receives, and group them into operations.
• defines the exchange pattern for the abstract messages: sequence, direction, and cardinality

<interface name="PictureDB" extends="..."?>
  <operation name="getResult"
	  pattern="http://www.w3.org/2003/11/wsdl/in-out">
      <input message="s0:name"/>
      <output message="s0:picture"/>
  </operation>
</interface>

WSDL 2.0 improves interfaces with inheritance: no overloading (if two operations have the same qname, then their components must be equivalent, i.e. their abstract model must be equal).