NServiceBus uses loosely coupled communications that depend on a contract between a publisher and a subscriber.  The contract is independent of either implementation and is based on one or more standard .Net class interfaces.  NServiceBus supports both publish/subscribe (pub-sub) and request/response messaging, but both messaging approaches occur in an asynchronous and disconnected manner.  In short, the calling application does not block after sending a message, it is a fire and forget mechanism.  In the case of pub-sub, the publisher does not expect a response; it is simply notifying subscribers of a system event.  This is the preferred mechanism of communication using NSB.