The Hurd is GNU's replacement for the various UNIX and Linux kernels.

The Hurd is firstly a collection of protocols formalizing how different
components may interact. The protocols are designed to reduce the mutual
[[trust]] requirements of the actors thereby permitting a more [[extensible]]
system. These include interface definitions to manipulate files and
directories and to resolve path names. This allows any process to
implement a file system. The only requirement is that it have access
to its backing store and that the principal that started it own the
file system node to which it connects.

The Hurd is also a set of servers that implement these protocols.
They include fie systems, network protocols and authentication.
The servers run on top of [[Mach]] and use Mach's [[IPC]] mechanism to
transfer information.