I always wanted to try Unix but I can't seem to find an ISO file or somewhere to buy it. Is Unix published for the whole wide world to use or is it a special OS only for high-class servers, mainframes and supercomputers?
Can I try it out? Does it comes in distros? This is like the problem I had with Linux: I was going around the Internet, wondering how to install Linux and looking for a Linux ISO when I did not know it came in distros.
Unix was originally a product, first developed in AT&T's Bell Labs. But today, the word “Unix”, except in historical context, means a family of operating systems, not a single product (similarly to “Linux” meaning a family of distributions, not a single product). This family has a somewhat complex history (see also Evolution of Operating systems from Unix).
It's difficult to say when this product ended, because the original code was licensed to a number of vendors, some of which still maintain their product. I believe the last product released by AT&T was Unix Time-Sharing System 10 in 1989. However by that time most Unix systems were actually modified versions of the AT&T code maintained by other companies such as Sun (SunOS, later renamed Solaris), Hewlett-Packard (HP-UX), IBM (AIX), etc. These three are still extant today, and they're directly derived from the AT&T code (although after over 25 years there probably isn't much of the AT&T era code remaining).
In addition to unix systems that are derived from the AT&T code, there are systems that don't contain any AT&T code but have a similar design and compatible user and programmer interfaces. The main families of such unix systems are BSD (FreeBSD, OpenBSD, NetBSD, Darwin/macOS etc.), Linux (many distributions) and MINIX.
Just to add to the confusion, UNIXⓇ is a trademark (actually, a family of trademarks: there are several versions) which does not designate a particular product: any product can use it as long as it passes a series of conformance tests. This unusual situation is a consequence of a long legal battle. Basically, a product can lay claim to one of the UNIX trademarks if it complies with the Single UNIX Specification, which describes user and programmer interfaces of the operating system (but not administration interfaces).
If you want to run a product that came directly from AT&T, you can run Unix V5, V6 or V7 on a PDP-11 simulator (the PDP-11 was a popular series of minicomputers in from the early 1970s to the early 1990s).
If you want to run a product based on code from AT&T, you can run OpenIndiana, which is based on the now-discontinued open source edition of Solaris (OpenSolaris). OpenIndiana is free software and runs on a PC. (It might not support as much hardware as Linux does though, but it can run in e.g. VirtualBox.) I believe that you can also download Oracle's Solaris for free for personal use, and it too can run on a PC. As far as I know, it isn't possible to run AIX or HP-UX on easily-available hardware or emulators.
If you want to run a product which has the UNIX brand, you can go through the official list. It includes several versions of Solaris (including PC versions), several versions of macOS, and a few uncommon Linux distributions. In a twist of fate none of the historical Unix products have the UNIX trademark, because they're too old and fail to meet some of the newer requirements for the UNIX brand.
If you want to run a product in the unix family of operating systems, Linux is one (or rather Linux is a subfamily, and each distribution is a Unix-like operating system).
FreeBSD is a free Unix-like operating system descended from Research Unix via the Berkeley Software Distribution (BSD). Although for legal reasons FreeBSD cannot use the Unix trademark, it is a direct descendant of BSD, which was historically also called "BSD Unix" or "Berkeley Unix".
Today Unix is a trademark, and an operating system can be certified as Unix if it complies to a set of certification procedures. There are several operating systems, not all directly based on original AT&T Unix, that have passed the certification and can call themselves Unix. Others just haven't bothered, but might pass. Linux for one goes it's own way whenever the developers (in the end, Linus Torvalds) deem the relevant standards misguided or directly braindead.
Unix is not just the kernel of the operating system. While Unix specifications cover system calls in detail, the specifications covers a set of libraries and a rich set of utilities and user tools too.
There are literally hundreds of Unix derivatives, and a large number of more or less clones. And the different lines have evolved over the last 40 or so years, together with the machines on which the systems run. Today's machines have little resemblance to the original PDP-7, or even the PDP-11 workhorse, or the very popular VAX-11 of yore.
In summary, "Unix" is a ill-defined term. If you want some feel of what it is all about, use one of the current BSDs or Linux. If you want a more specific version, you'll have to hunt it down, perhaps at The Unix Heritage Society.
I'm posting this because whilst the other answers are good, I feel any discussion like this really needs to include a reference to the POSIX standard.
Unix isn't a thing, it's a family. A bit like mammals - a mammal has a certain set of features that make it a mammal - like 4 'legs', warm blooded, endoskeleton, etc.
But there's a big difference between an elephant and a seal. (Or a penguin!)
Unix is much the same - there is no 'true Unix' there are just a bunch of things in the family, that have common features.
From: http://www.levenez.com/unix/
We have a family tree that is quite long and complicated - it starts with Unics in 1969, and ... well, branches a lot from there.
Linux is a descendent of that family tree. It also implements POSIX, and therefore is Unix. You have some other major branches at the moment - the bigger examples are:
There's been a degree of crossbreeding - Linux is proving quite a successful beast, thanks to its links to open source. (Unix has always had a cordial relationship with Open Source, but most distributions historically have been vendor supplied/supported/patched.). But BSD ... well, Apple adopting it certainly hasn't hurt any. It's also quite popular in a lot of embedded systems - firewalls and the like (I think NetApp is based on BSD too).
Likewise - historically a lot of the big players had their own hardware/processors and OS.
So the long and short of it is - if you've already done Linux and got a distribution installed that you like - you have already used Unix, and all is good. If you really want you could give some other variants a whirl, and see how they differ (and yet how similar they are too!)
So for example, you can get Solaris for x64 which is a System V, FreeBSD
But you'll have some similarities (POSIX standard, and things that operating systems have 'evolved' from each other) and some differences (AIX keeps 'getting' me with the ODM).
