[nameismypw]

Hurd is part of the GNU system; it's the intended kernel for the GNU userspace. Linux has been used as an intermediary because Hurd isn't ready for widespread use. But the idea has always been that Hurd would replace Linux and create a GNU system that is entirely under the FSF.

Beyond the philosophical differences, another difference is that Hurd is a microkernel and Linux is monolithic. Hurd can be considered a research project for exploring microkernels. The most well-developed microkernels are not open source or free.

[jcranmer]

> But the idea has always been that Hurd would replace Linux and create a GNU system that is entirely under the FSF.

I don't think that's the case. The FSF has long been satisfied that Linux meets its goals for an OS kernel, and the Hurd project has changed somewhat from "this will be the final piece of the GNU operating system" to "this is something we're working on to explore microkernel design." I think its developers have given up on it ever replacing Linux, as it's very behind in hardware support, and the gap is only ever growing rather than shrinking. For example, it doesn't support multicore or 64-bit userland (in a time when projects are pulling 32-bit userland support!).