The GNU Hurd is the GNU project's replacement for the Unix kernel. It 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).
Mach
Mach is a so-called first generation microkernel. It is the microkernel currently used by the Hurd.
Mig
The Mach Interface Generator (MIG) is an IDL compiler. Based on an interface definition, it creates stub code to ?invoke object methods and to demultiplex incoming messages. These stub functions conveniently hide the details of Mach's IPC and port machinery and make it easy to implement and use Mach ?interfaces as remote procedure calls (RPC): by using the stub functions, the client programs can call remote procedures more or less like any other C function.
Development
The GNU Hurd is under active development. Because of that, there is no stable version. The latest release is GNU Hurd 0.8.