Unix (or UNIX, if you prefer) is a generic term for a 70s-vintage operating system which until recently was primarily used in academic and business settings. I won't attempt to give you a history of Unix--you can find that elsewhere. Nor will I try to cover all the variants of Unix. Instead, I will cover versions of Unix which are available and useful for theologians/clergy and their flocks. I won't try to be exhaustive, either, but instead present what seems most useful. My primary interest here is the so-called "free" versions of Unix, mainly Linux and FreeBSD. I may also mention proprietary versions of Unix if they are useful to the average theologian. You can get cheap versions of Solaris and SCO OpenServer and Unixware, but these won't be quite as useful.
Linux has had a lot of media coverage lately, but finding a clear and comprehesive description of Linux can still be difficult. That's because Linux is a large subject to cover in a short sound bite, and because (I suspect) some of the media-types who are covering Linux don't understand it. Since you're here for Linux information, I'll try my best to tell you just what Linux is!
The term "Linux" can be used in two different senses, a narrower and a wider sense. In a narrower sense, Linux is an operating system kernel, a computer program which provides useful and important services to other computer programs. The Linux kernel gives other programs the ability to do such things as write to the screen, work with disk files, cooperate with other programs, etc. Your desktop computer needs an operating system to give it these abilities. Some operating systems do these things well, and some don't. MS-DOS did some of these things, but not well. Windows 95/98, built on MS-DOS, does these things, but has some of MS-DOS's limitations and problems. Linux does all all these things, and more, and does them quite well. Linux can make your older and slower computer seem years younger!
Linux was designed by a Finnish college student, Linus Torvalds. Linus wanted to run the Unix operating system on his 386 PC, but couldn't afford to pay the price (for a single user, Unix cost around $1000 in 1990). A Unix workalike, named Minix, was commonly used in college courses on operating systems. Linus wrote an operating system which also worked like Unix, and released it on the internet in 1991. Since then, numerous programmers have extended Linux to work better, to work more like Unix, and to work with most PC-type hardware available.
Other programmers have spent time extending Linux to include more than just the operating system kernel. Linux (in the wider sense) is a whole distribution of software, a collection of hundreds (even thousands) of software packages which can handle the vast majority of your computing needs. Some of that software is age-old traditional Unix software and some of it is new. Programmers all over the world write software and give it away for the good of other users, simply because they like to write programs.
As a Linux user, you reap the benefit of this sharing attitude. For a few dollars you can buy a Linux distribution (popular distributions include Red Hat, Mandrake, SuSE and Slackware) which contains hundreds of megs of software for just about any imaginable application, and you can use and copy it without worrying about copyrights.As a side benefit, it's generally faster and more stable than Microsoft Windows. Hmmm, faster, more stable and much cheaper. No wonder Linux has been getting so much attention!
Click here for another overview of Linux, and here for some reasons to use Linux.
FreeBSD is another free version of Unix, one which is a direct descendant of the original Unix rather than a freshly written clone, as is Linux. Although it is more stable than Linux and is used to run some of the Web's biggest sites (such as Yahoo), FreeBSD has received less attention than Linux in the press. It is nevertheless a solid platform worth considering for your use.
FreeBSD also has two siblings, OpenBSD and NetBSD, which are also descendents of Berkeley Unix. OpenBSD has a reputation for being highly secure, while NetBSD is known for working on a wide variety of hardware platforms, rather than only running on Intel.
These are commercial versions of Unix. Solaris is produced by Sun Microsystems and runs a lot of high-end servers. OpenServer and UnixWare are both products of SCO. You can get personal use versions of each of these for relatively low cost (less than Windows). Features vary. They're included here for the sake of completeness. If you want to do serious day to day work with Unix, you're likely better off with Linux or FreeBSD. If you're a geek, though, you might want to check these out (UnixWare does have some nice features).