GNU is a modern, commercial-grade operating system consituted of software components that respect the freedoms of computing device users.
The GNU Project recently celebrated the 30th anniversary of its founder, Richard Stallman's, announcement -- in a Unix newsgroup -- of his intention to build a complete, free operating system. The canonical tale tells of Stallman's desire to fix a printer driver and being denied access to the source code. As the story goes, the refusal by the author of the printer driver to share the source so incensed Stallman that he launched the project to build the GNU System.
The experienced hacker made three fateful, early decisions about his system, 1. it would be built using Free Software, 2. because of its popularity it would be modeled after Unix. Hence the name (using a common convention in hacker circles), GNU's Not Unix (GNU) -- the word "GNU's" is a contraction of "GNU is" -- and 3. to put off coding the kernel, because he believed he could use another kernel already in development.
Soon, he set out to build the basic components. Because he (and others) had already written the system's text editor (Emacs) he focused on the compiler, GCC; the core utilities, coreutils; and a handful of other programs. Importantly, he also took the time to write the guiding document of the project, the General Public License (GPL), which ensures that the system's software will always remain free.
Within a few years a young hacker, Brian Fox, began work on the system's shell. Which -- because it would be a compatible superset of the standard Unix shell written by Stephen Bourne -- he called the Bourne-Again Shell, or bash. The shell proved to be so popular that it is not only the default shell for GNU but for many other Unices, including Mac OS X.
After almost a decade of development and still without a kernel, a computer science student in Finland, Linus Torvalds, announced his desire to build a Unixoid system that would run on the personal computer clones that had recently become popular. Because he chose to release his kernel under the GNU GPL, it, Linux, became the default kernel of the GNU OS.
But it's not the only kernel. (Part of what differentiates GNU from all other operating systems is its flexibility, which extends right down to the kernel level. Because GNU was created to ensure users' freedoms, one has the ability, to not only modify every component of the system [even the kernel] but, to replace it with another.) GNU developers have been busy with writing their own kernel. They chose a microkernel design and are currently writing the protocols and services to go with their version of the Mach microkernel. They call it Hurd. Although they have a ways to go (and they could use your help), version 0.5 was released just last week.
The only thing left to do was build a package manager and distribute the system in aggregate. GNU Guix was launched at the end of last year to do just that. The maintainer's road map indicates that the project will be self-bootable (ready for mainstream use) by the end of this year.