HURD

Hurd is a collection of servers that run on the Mach microkernel to implement file system, network protocols, file access control, and other features that are implemented by the Unix kernel or similar kernels (such as Linux).

First, a little background...
The goal of the GNU project is to create a Free Unix-like OS from scratch and name it "GNU". Richard Stallman started the whole project, and implemented many parts of the OS himself in the beginning.

Much of the way through the project, RMS found that the only major thing the project lacked was a kernel. At just about that time, Linus Torvalds wrote Linux, plugged it into the rest of GNU, and out popped an operating system (the one many folks refer to simply as "Linux" these days).

Thus, Linus and Company went ahead and used GNU + Linux as their OS, and RMS went ahead and chose the not-yet-implemented HURD + Mach as the future GNU kernel.

With this back-story in mind, you can see why RMS prefers that folks call the OS "GNU/Linux" rather than simply "Linux".

From the HURD web page : 
 * Hurd 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).

 HURD is supposed to be able to run with other microkernel's besides Mach, such as L4.

What "HURD" (the acronym?) stands for, also from their web page: 
 * `Hurd' stands for `Hird of Unix-Replacing Daemons'. And, then, `Hird' stands for `Hurd of Interfaces Representing Depth'. We have here, to my knowledge, the first software to be named by a pair of mutually recursive acronyms.



Finally, a historically interesting quote from Linus:


 * "If the GNU kernel had been ready last spring, I'd not have bothered to even start my project"