I'll make an attempt at an answer despite not being entirely sure:
- Service Interface
This is what clients of the service see. Something that describes the operations made available by a service and how to communicate with the service.
For a SOAP web service, this would typically be a WSDL.
- Endpoint interface.
If the web service in question is implemented in Java, then this is a Java interface specifying which web service operations are available in the endpoint implementation class.
Clients of the web service never see this interface directly.
Hope this helps!