Mac is UNIX. Registered UNIX systems: * AIX * HP/UX * Mac OS X and Mac OS X Server * Reliant UNIX * SCO * Solaris * Tru64 UNIX * z/OS * NCR UNIX SVR4 * NEC UX/4800 * SGI IRIX 6.5 Linux is made to work like UNIX, but is not UNIX registered. It is among a group of OSes called "UNIX-like". Many people get confused by the similarity in spelling and pronunciation, so they think Linux must be a type of UNIX. Not so. Apple used an OS (version 1.0 to 9.2.2) that was not even closely related to UNIX from 1984-2001, and that keeps many people from realizing that OS X uses the X for three reasons: X=ten, X for UNIX, and X for NeXT. Windows is not even closely related to any of the UNIX or UNIX-like OSes. It was originally designed as an overlay for MS-DOS, and still has many of the details of the DOS system. It is designed for BIOS compliance, a huge limitation that Microsoft has been trying to walk away from for the past ten years, but it seems the dozens of PC makers won't go along with the change to EFI that allows Mac OS and Linux (when installed on an EFI system) to out-perform Windows in most measurements. It would take a thousand words to explain the fine details that make these different. Some general concepts are: -- Most Linux OS varieties are available free. That is a big attraction for some people. Mac OS costs US$29.95. Windows costs around US$200 -- The largest number of apps are available for Windows OS. The next largest number are for Mac OS. Fewer are available for Linux. Very few home use applications are designed for UNIX systems except the Mac system. Most UNIX use is for government, educational facilities, and scientific research. As far as what people install, it seems most people do not install anything. They buy a computer that has an OS already installed. They don't question it. If the OS stops working, they take the computer to a shop and pay to get the same or an updated version of the same OS installed again. They already have a system DVD that came with the computer, so they see no need to buy some OS DVD tey are not familiar with, erase their documents, and install a whole new experience. The computer is a tool, not a toy to them, so it isn't fun as a piece of hardware. The Internet is fun, movies are fun, but not the hardware or the OS. There is an old saying that a computer nerd uses a computer to use a computer. The number of geeks using computers is far less than the non-geeks. The geeks just make a lot more noise, so we think they represent most people. I am sure if shops started selling only computers with Linux installed, most of their customers would go elsewhere to buy a computer. The average person has no use for Linux, because it can't play most games, can't run Microsoft Office, can't synch to an iPod, can't synch to an iPhone, can't play QuickTime movies, and can't play Shockwave Flash. There are hack-style ways to get around a couple of these limitations such as running another OS in Wine, but the average person has no interest in bending over backward to make Linux work almost as well as Windows or OS X. That's not to say Linux isn't a powerful, flexible, secure system. It just is a nitch system that the average person has no interest in. That's why it hoovers around 1% of the home market.