NServiceBus, the heart of the system, is a messaging and workflow framework that helps create distributed systems that are scalable, reliable and easy to modify. It supports various messaging patterns, handles long-running business processes in the form of sagas and provides abstraction over multiple queueing technologies. While most queueing technologies try to make guarantees regarding 'at least once' or even 'exactly once' delivery, they often fall short of this promise. NServiceBus contains mechanisms to automatically solve intermittent delivery problems by retrying messages and falling back to an error queue where they can be exposed by the rest of the Platform for human intervention (ServiceControl, ServicePulse).
Moreover, NServiceBus is thoroughly extensible. It can be tailor it to many requirements, compatible with many technologies and many elements of the system can be replaced with custom implementations.
RavenDB Persistence: Installing RavenDB
Article outlines various ways of installing RavenDB in different versions of NServiceBus. As of Version 5 it is mostly obsolete due to the fact that RavenDB is no longer part of the core.