Synchorous.. Commonly used in web environments. A client makes a request and waits for the response. example: Asynchorous.. A client will send a request, disconnet, then may come later and pick up the response by supplying the given uniqueID. I once read in Axis mailing list they have a sample on this, but cant find it now. I guess BPEL is making use of Asyn calls (have to verify).
Async WS's (i.e. document/literal style web services) are able to invoke a service and carrying on processing (unlike RPC which has to wait for a response). Yes a reply is sent back but it is purely a receipt to let the client know the service received the request successfully. In some cases this reply can be ignored.