Contemporary Unix Products, (from UNIX System Administrator's Handbook
   by Evi Nemeth, et. al.

   In this book, we use the abbreviations ATT and BSD to identify
   versions of UNIX as being more ATT-like or more Berkeley-like,
   respectively. By virtue of lineage, Novell's UnixWare and BSDI's
   BSD/OS are the most generic systems relative to our terminology.

   Sun Microsystem's Solaris 2.X is an ATT UNIX with many extensions.
   SunOS, an older operations sytem from Sun, is a BSD-based system, also
   with many extensions. Sun publications sometimes refer to SunOS as
   Solaris 1.X. Do not be alarmed; this is just Sun's Way of editing
   history to serve current marketing needs. Due to popular demand, Sun
   has agreed to support both operating systems, at least for a while.

   DEC uses the OSF/1 standard, which is in turn based on the Mach
   operating system developed at Carnegie-Mellon University. Mach is
   derived from BSD (though the kernel and UNIX interface are structured
   differently), and OSF/1 systems behave much like BSD machines from an
   administrative point of view. NeXT Computer's NEXTSTEP is also an
   extended Mach, though it is not of the OSF/1 ilk. You might also  
   encounter DEC machines that run DEC's previous UNIX, called Ultrix; it
   is more or less a vanilla BSD system.

   IBM, backware as ever, flogs a weirdo product called AIX, which they
   deny stands for "Ain't UNIX." It's, ah, UNIX-compatible? Silicon
   Graphics provides IRIX, which is similar to ATT. Early versions had
   all the good BSD extensions, but IRIX ahs been steadily creeping
   towards ATT-land ever since. SCO, the Santa Cruz Operation, sells a
   popular but perverse PC UNIX based on al old version of ATT UNIX,
   System III.2. It ahs been extensively modified. HP's system is known
   as HP-UX; it is mostly ATT-ish but with odd surprises of its own.

   Several free UNIX systems are available for PC hardware. The most
   popular is called Linux. It was originally written by Linus Torvalds
   of Helsinki, but now has a large and growing developer community. It
   requires a 386 PC with 4MB of memory and a 40MB hard disk. Linux
   internals and system administration are BSD-ish (probably closest to
   SunOS); its programming interface is more like ATT.

   NetBSD, 386BSD, and FreeBSD are PC UNIX implementations based on Net/2
   from Berkeley. The require similar hardware.