WS-BPEL is very much a subset of what can be represented using WF. Think of WF as a "graph of activities" that can be transformed to virtually any workflow standard. I'll try to post a similar comparison between WS-BPEL 2.0 and XPDL 2.0 later this week.
Workflow start or WebServiceReceive
Workflow finish or WebServiceResponse
Delay , Suspend