Debian GNU/Hurd vs Debian GNU/kFreeBSD
Debian GNU/Hurd and Debian GNU/kFreeBSD are UNIX-like operating systems composed of free (libre) software. Debian GNU/Linux is one of the 5 most popular Linux distributions but in these iterations Debian replaces the Linux kernel with a different kernel. The operating system kernel is a piece of software that communicates with computer hardware, allowing other software to ignore the details of what hardware is being used.
[edit] Components
Debian GNU/kFreeBSD uses the kernel of the FreeBSD operating system (referred to as "kFreeBSD" for short), and Debian GNU/Hurd uses the Hurd, which is a collection of daemons that implement kernel-like functionality. Both projects use GNU's core utilities, which provide basic operating system functionality (a base on allowing you to use other applications).
[edit] Debian Software Compatibility
Debian GNU/kFreeBSD supports about 85% of the debian software collection. Debian GNU/Hurd supports about 50%. Debian GNU/kFreeBSD will work with most hardware that is compatible with Debian GNU/Linux. Debian GNU/Hurd is not recommended for people who want compatibility with the widest variety of hardware.
[edit] Microkernel / Monolithic Kernel
Debian GNU/Hurd uses a microkernel where the rest of the kernel-type functionality is provided by small programs that communicate with the kernel. Debian GNU/kFreeBSD uses a monolithic kernel. In theory a microkernel has technical advantages, but monolithic kernels are compatible with a much larger portion of the hardware and software that people use at present. Theoretically a microkernel may make it easier to make software both secure against attacks and reliable. However, most software developers focus on the established monolithic-kernel platforms, so it's not clear whether the theoretical advantages of microkernels work in practice. Debian GNU/Hurd project states that their software doesn't provide the stability and performance users expect for production (day-to-day) use.
