So, there are lots of different versions of Unix out there: HP-UX, AIX, BSD, etc. Linux is considered a Unix clone rather than an implementation of Unix. Are all the "real" Unices actual descendants of the original? If not, what separates Linux from Unix?
That depends on what you mean by “Unix”, and by “Linux”.
UNIX is a registered trade mark of The Open Group. The trade mark has had an eventful history, and it's not completely clear that it's not genericized due to the widespread usage of “Unix” refering to Unix-like systems (see below). Currently the Open Group grants use of the trade mark to any system that passes a Single UNIX certification. See also Why is there a * When There is Mention of Unix Throughout the Internet?.
Unix is an operating system that was born in 1969 at Bell Labs. Various companies sold, and still sell, code derived from this original system, for example AIX, HP-UX, Solaris. See also Evolution of Operating systems from Unix.
There are many systems that are Unix-like, in that they offer similar interfaces to programmers, users and administrators. The oldest production system is the Berkeley Software Distribution, which gradually evolved from Unix-based (i.e. containing code derived from the original implementation) to Unix-like (i.e. having a similar interface). There are many BSD-based or BSD-derived operating systems: FreeBSD, NetBSD, OpenBSD, Mac OS X, etc. Other examples include OSF/1 (now discontinued, it was a commercial Unix-like non-Unix-based system), Minix (originally a toy Unix-like operating system used as a teaching tool, now a production embedded Unix-like system), and most famously Linux.
Strictly speaking, Linux is an operating system kernel that is designed like Unix's kernel.
Linux is most commonly used as a name of Unix-like operating systems that use Linux as their kernel. As many of the tools outside the kernel are part of the GNU project, such systems are often known as GNU/Linux. All major Linux distributions consist of GNU/Linux and other software.
There are Linux-based Unix-like systems that don't use many GNU tools, especially in the embedded world, but I don't think any of them does away with GNU development tools, in particular GCC.
There are operating systems that have Linux as their kernel but are not Unix-like. The most well-known is Android, which doesn't have a Unix-like user experience (though you can install a Unix-like command line) or administrator experience or (mostly) programmer experience (“native” Android programs use an API that is completely different from Unix).
For all intents and purposes, a typical modern Linux distribution (Ubuntu, Debian, Red Hat, Fedora, Slackware, etc) is a Unix, but strictly speaking, no system can claim to be Unix without being certified, so instead people say they are Unix-like. They are inspired by Unix, and carry on its culture.
This also applies to BSD systems.
Mac OS X is certified Unix, so it's Unix both in name and indeed. (and it's actually based on BSD).
It should be noted that since Linux itself is just a kernel, it can be used to build non-unix-like systems (such as Android).
Linux is more 'Unix-like' so yes simplistically you could call it a clone, the same is true for BSDs (although admittedly BSDs could be considered closer to Unix than Linux).
The main thing that gives Linux the Unix-like title is the fact that it is nearly fully compliant w/ POSIX (Portable Operating System Interface [for Unix]) standards that have built up over time.
The other key thing, is the inheritance of code etc, as demonstrated on Wikipedia, Linux does not actually originate from Unix sources, instead it is pretty much built from scratch (this is disputed however).
So essentially, the main thing that separates Unix from Linux is the ancestry and the standards that it meets.
The choosen answer explains it quite exhaustively, but you would have to watch a whole documentary to know all the details between the UNIX initial developement, and how Linux went out.
First you have to consider that Linux is the kernel, which was made by linus and other programmers. He chose to release it with the GPL, which by the time was a license made by the FSF, (RMS and other folks), who were also developping their own kernel and GNU.
What we usually call Linux is, at first, the kernel, plus all other tools originally coming from the GNU project. Those two project are historically different, because at the time, the kernel from the FSF folks was abandonned to favor the Linux kernel, which was much better.
I remember there is a documentary, you should really watch it, I find it important for your programming culture.
Another thing you have to consider when thinking about "*nix", is also everying involving POSIX and other architecture-standardised stuff. It's subject of OS design/research, but it defines precisely how the involved OSes works, and is crucial when you have a kernel working well with its tools.
There are lots of great answers already but I thought the views of the great Dennis Ritchie (co-creator of Unix) would be an interesting complement to the other more technical answers.
It seems that Dennis Ritchie considered Linux to be a legitimate Unix derivative. In a 1999 interview for LinuxFocus.org, he was asked,
what is your opinion about all the Linux momentum, and the decision of many companies to start developing software for it?
His response to the question was
I think the Linux phenomenon is quite delightful, because it draws so strongly on the basis that Unix provided. Linux seems to be the among the healthiest of the direct Unix derivatives, though there are also the various BSD systems as well as the more official offerings from the workstation and mainframe manufacturers. I can't help observing, of course, the "free source" Unix-derived world seems to be suffering from exactly the same kind of fragmentation and strife that occurred and is still occurring in the commercial world.
Historically Linus Torvalds has created an improved clone of Minix OS. The development of Linux is not focused on a specific platform and customer base, and Linux developers have a variety of experiences and perspectives. Such OS as HP-UX, AIX, Solaris and others are mostly sharpened for vendors.
In the Linux community there is no strict standard set of tools or environments. This lack of standardization leads to significant inconsistencies within Linux. For some developers, the ability to use the best achievements of other operating systems is a plus, but it's not always convenient to copy UNIX elements on Linux, for example, when the device names inside Linux can be taken from AIX, while the tools for working with the file system are focused on HP-UX. Incompatibilities of this kind are also found between different Linux distributions.
Unix, originally UNICS (UNiplexed Infomation and Computing Service).
Unix (trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix. Development started in 1969 and announced outside Bell abs in October 1973.
Variety of both academic and commercial variants of Unix from vendors such as the University of California, Berkeley (BSD), Microsoft (Xenix), IBM (AIX) and Sun Microsystems (Solaris).
- BSD(Berkeley Software Distribution) releases provided a basis for several open source development projects that are ongoing, e.g., FreeBSD, OpenBSD, NetBSD, Darwin, and TrueOS.
- Xenix is a discontinued version of the Unix operating system for various microcomputer platforms, licensed by Microsoft.
- AIX(Advanced Interactive eXecutive) developed and sold by IBM for several of its computer platforms.
Linux is a family of free and open-source software operating systems built around the Linux kernel.The defining component of Linux is the Linux kernel, an operating system kernel first released on September 17, 1991 by Linus Torvalds.
Linux is packaged in a form known as a Linux distribution (or distro for short) for both desktop and server use. Some of the most popular and mainstream Linux distributions are Arch Linux, CentOS, Debian, Fedora, Gentoo Linux, Linux Mint, Mageia, openSUSE and Ubuntu, together with commercial distributions such as Red Hat Enterprise Linux
The user interface, also known as the shell, is either a command-line interface (CLI), a graphical user interface (GUI), or through controls attached to the associated hardware, which is common for embedded systems. For desktop systems, the default mode is usually a graphical user interface, although the CLI is commonly available through terminal emulator windows or on a separate virtual console.
- CLI shells are text-based user interfaces, which use text for both input and output. The dominant shell used in Linux is the Bourne-Again Shell (bash), originally developed for the GNU project.
- GUI shells are K Desktop Environment (KDE), GNOME, MATE, Cinnamon, Unity, LXDE, Pantheon and Xfce, though a variety of additional user interfaces exist. Most popular user interfaces are based on the X Window System, often simply called "X".
GNU is an operating system and an extensive collection of computer software. GNU is composed wholly of free software most of which is licensed under the GNU Project's own GPL. GNU is a recursive acronym for "GNU's Not Unix!", chosen because GNU's design is Unix-like, but differs from Unix by being free software and containing no Unix code. Development of the GNU operating system was initiated by Richard Stallman while he worked at the Massachusetts Institute of Technology (MIT) Artificial Intelligence Laboratory in 1983.
Basic components include the GNU Compiler Collection (GCC), the GNU C library (glibc), and GNU Core Utilities (coreutils), but also the GNU Debugger (GDB), GNU Binary Utilities (binutils), the GNU Bash shell and the GNOME desktop environment.
Linux is a Unix-Like Operating System developed by Linus Torvalds and thousands of others.
BSD is a UNIX operating system that for legal reasons must be called Unix-Like.
OS X is a graphical UNIX Operating System developed by Apple Inc.
Linux is the most prominent example of a "real" Unix OS. It runs on anything and supports way more hardware than BSD or OS X. An interesting quote I found when I was comparing BSD and Linux:
Linux is what you get when a bunch of PC hackers sit down and try to write a Unix system for the PC. BSD is what you get when a bunch of UNIX hackers sit down and try to port a Unix system to the PC.
BSD is more like a Unix OS than Linux is. Also notable is that Apple makes use of BSD and Linux components. Apple Uses APT from Debian and Ubuntu on the iOS and OS X platforms. And it is based on BSD. (The kernel though is Darwin, which is it's own kernel. Beastie the platypus is the Darwin mascot because he is a mix between Beastie from BSD and a Platypus.)
If you want a "real" Unix operating system (One that runs on anything and supports lots of hardware), try Linux.
If you want lower-end hardware support and headaches (I know I'll get a ton of hate but I don't care), use BSD.
If you want to spend $1000+, use OS X and iOS. (Again I'll probably get a ton of hate.)
I'm a long-time Linux User, having used it off and on from the 90's to early 2000's and then quit using it for awhile but started using it again around mid 2012 as my permanent OS so I can recommend it to anyone who wants to try something other than Windoze.