All cloud services communicate via web services to on-premise data or to other cloud services. Even though products such as the Salesforce cloud is normally kept in the cloud versus on-premise, there may still be data sent to Salesforce and kept on-premise. There could be many reasons why some data could be kept on-premise and not stored in the cloud; some reasons may be that there is proprietary data that a company wishes not to store off-premise, or data for security reasons may need to be kept on-premise.

Salesforce provides WSDL interfaces to their cloud by downloading their WSDL and client certificate guide, https://help.salesforce.com/HTViewHelpDoc?id=dev_wsdl.htm&language=en_US. A workflow rule in Salesforce would have to be created to define the data fields that have to be defined. See the tutorial found at http://www.ramonsmits.com/2013/04/08/receiving-salesforce-notifcations-with-nservicebus.html as an example. We have already covered setting up NServiceBus...