GNU/Linux is a monolithic kernel while the HURD is based on GNU Mach, the microkernel of the GNU system. The Hurd is a collection of servers that run on the Mach microkernel to implement services. See :
http://www.gnu.org/software/hurd/hurd.html
http://www.gnu.org/software/hurd/gnumach.html
> LINUX: (or GNU/Linux) Operating system based on Linus Torvalds' free
> Unix-like kernel
>
> HURD: (or GNU/Hurd) Operating system based on the FSF's work and the
> Mach microkernel
>
> I'm not terribly clear on what the functional differences between
> Linux and the Hurd are, only that they're Not The Same (TM). The FSF
> wants you to call Linux GNU/Linux since it's the GNU userspace on top
> of the Linux kernel, as distinguished from GNU, which is the GNU
> userspace on top of the GNU Hurd kernel.
>