What are the essential differences between publishing a message using Bus.Publish and sending a message using Bus.Send? I am looking to understand how they differ and also when I should choose to use one over the other.
Publishing is used to notify multiple Subscribers of a particular event. A Publishing endpoint will have subscription storage to identify where to send messages to. Sending is typically used to issue a command to an endpoint. A command is telling the endpoint to do something and should not expect a reply(although you sometimes do want a reply and NSB supports this).
The reason you do not see a destination for Send() is that you specify the destination via configuration. In your app.config you will map message types(a whole assembly or a class) to a destination. When you do so, you do not have to provide the destination.