what is exact meaning of portname

[Raj Kumar Bindal]

In main web service ,it is written :
@WLHttpTransport(contextPath="conv",
serviceUri="ConversationalService",
portName="ConversationalServicePort")

and in client web service:

@ServiceClient(
wsdlLocation="http://localhost:7001/conv/ConversationalService?WSDL",serviceName="ConversationalService",
portName="ConversationalServicePort")

I know portname is the port on which we will be invoking the web service,but why is this needed.I think if the need of portname is only to assciate a particular main web service with a client web service,then service name is already for that.
I am not clear.please tell me the use of port name??