HURD is a kernel, just like LINUX. It's just a different kernel.
They follow a few different technical methods, for example, HURD is a microkernel, LINUX is a monolithic kernel.
But basically, HURD was started before LINUX, but since linus was bored with waiting for a good *nix on his PC (HURD had no release date set), he made LINUX.
LINUX took off and people ignored HURD, so it was never really "released". It's still in development by a couple of people though.