The Hurd builds on top of a microkernel that is responsible to provide the most basic kernel services -- coordinating access to the hardware: the CPU (through multiprocessing?), RAM (via [memory management]?), and other various devices for sound, graphics, mass storage, etc. Currently [GNU Mach]? is used as the microkernel. Projects exists to port the Hurd to other microkernels.
There are other Unix-like systems running on top of Mach (including Lites?, and MkLinux?), implemented as a single so called "server". In effect they replace a [monolithic kernel]? with two parts (microkernel and server). The Hurd instead consists of multiple servers working together -- it is a "Hird of Unix-Replacing Daemons". ("Hird" stands for "Hurd of Interfaces Representing Depth".)
A number of traditional Unix concepts are replaced or extended:
Under Unix every program running has an associated [user id]?, which normally corresponds to the user that started the process. This id largely dictates the action permitted to the program. No outside process can change the user id of a running program. A Hurd process, on the other hand, runs under a set of user ids, which can contain multiple ids, one, or none. A sufficiently privileged process can add and remove ids to another process. For example there is a password server that will hand out ids in return for a correct login password.
Regarding the filesystem, a suitable program can be designated as a translator for a single file or a whole directory hierarchy. Every access to the translated file, or files below a hierarchy in the second case, is in fact handled by the program. For example a file translator may simply redirect read and write operations to another file, not unlike a Unix symbolic link. What mounting is to Unix, the Hurd achieves by setting up a filesystem translator.
The Hurd requires a multiboot-compliant bootloader, GRUB for example.
At this time, there is a single distribution of the Hurd in preparation, which has not yet been officially released outside of interested testers and developers. That distribution is Debian GNU/Hurd.
See also: