Interview with Hurd developer Marcus Brinkmann | Wikinerds Portal:
Marcus Brinkmann: The Hurd is a multi-server system, running on top of a microkernel. This means that separate features of the kernel, like the network stack, authentication, or the filesystem, are implemented in different user-space processes, and not in the kernel. There are only few real multi-server systems out there, most other microkernel based systems you have heard about actually only run a single server on top of the kernel and thus are very similar to a monolithical kernel like the Unix kernel and its imitations.
The Hurd design goes one step further ahead of other multi-server systems and puts an emphasis on the user’s freedom to extend and enhance the operating system with new features. There is no mutual trust required for the individual components to cooperate. So, for example, the user can start their own filesystem services and attach (“mount”) them to the system’s file system hierarchy. Or a user can ignore the process servers services and start their own process management facilities, that is dissimilar to POSIX.
[Updated] Recommended reading: AST‘s “Modern Operating Systems” (for somewhat outdated implementation details: “Operating Systems Design and Implementation“) and the papers about the L4 microkernel.