For the vast majority of people, their first experience on a computer operating system (OS) is Microsoft WindowsR. Microsoft Windows runs on approximately 85 percent of computers connected to the Internet, but other operating systems besides Windows are out there, each with its own strengths and weaknesses.
Around 14 percent of computers connected to the Internet have some version of the Mac OS running on them. The most popular of these is the Snow Leopard (OS x v 10.6) with around 43 percent of Mac users running this version. Lion (OS x v 10.7) follows closely behind at around 40 percent. The other 20 percent is spread mostly between the Leopard (OS x v 10.5), Tiger (OS x v 10.4) and Mountain Lion (OS x v 10.8) editions, leaving only a quarter of 1 percent running any other version of the Mac OS x including server software.
The MAC OS keeps the core code (kernel) of the OS separate from the programs running on the system, so it is less prone to fall prey to viruses and other malware than the Windows OS - an ideal feature for those who are security conscious. Macs are also particularly good for graphic design work and image manipulation, making them popular in the creative industries. However, most software is developed for use on Windows PCs. Getting programs that will run on the Mac can be a challenge and is a real downside to the OS.
The only other computer operating system with any volume of usage on the Internet isn't really a single operating system, but a group of systems based on the Linux/Unix kernel. Linux and Unix, collectively known as *Nix, are popular operating systems for servers. Several 'flavors' of *Nix or *Nix-like systems exist such as Ubuntu, Solaris and GNU, but only Linux has enough installations running and connected online to show up on statistical analysis listing every OS with more than 0.1 percent market share. Even then, it only manages to amass around 0.85 percent.
The same security advantage the Mac OS enjoys applies to *Nix systems, only more so because of the lack of malware targeting the system. They are popular with programming enthusiasts because the majority of the systems are open source, meaning the code is available to read and accessible to modify.
Windows programs won't run on a *Nix based system just as they won't run on a Mac. However, *Nix's amateur programming community is an enthusiastic one. If you want to run a Windows program on a *Nix machine, chances are someone has built a clone or emulator to mimic the program and allow it to run on a *Nix OS. These programs may be of variable quality, leading to an interesting paradox; while the *Nix OS may be the most stable, the programs available on it tend to be the most unpredictable, unless they have a large community working on them.