Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
Windows is by far the most commonly used operating system, with over 95% market share. The next biggest operating system is Apple's OS X. The second most popular alternative to Windows is Linux. Linux is a free, open source operating system which can run on any system. It is redistributed in a number of different forms, the most common being the Ubuntu distribution. Linux has a comparatively small user base, and it has traditionally been considered difficult to use, however it has gotten better over time, and is fairly easy to install. Linux is supported by a large developer community, and it isn't hard to find help when you need it.
The Differences in Operating Systems
Over the recent decade or so, there has been some convergence of the operating system market. Several years ago, the three major operating systems - Windows, Mac, and Linux, were all very different, and had very steep learning curves.