The GNU Hurd is the GNU project's own UNIX kernel. According to their web page, "It is a collection of servers that run on the Mach microkernel to implement file systems, network protocols, file access control, and other features that are implemented by the UNIX kernel or similar kernels (such as Linux)."
GNU Mach itself is the microkernel approach of the GNU system. In other words, a "microkernel provides only a limited functionality, just enough abstraction on top of the hardware to run the rest of the operating system in user space. The GNU Hurd servers and the GNU C library implement the POSIX-compatible base of the GNU system on top of the microkernel architecture provided by Mach."
As time goes by, both the Hurd and GNU Mach probably will be ported to other hardware architectures. Currently it only runs on Intel 32-bit architectures. Nevertheless, its release cycles are much slower than those of Linux or BSD. It is more a proof-of-concept test bed for kernel specialists.