Linux is used to power the servers that host millions of websites. Google has been able to deploy thousands of Linux-based servers very inexpensively in order to provide reliable web services. While Linux can be used for a variety of applications (including personal desktops), it is primarily used on servers.
Microsoft Windows is (by far) the most popular operating system. Most personal machines run versions of Windows (primarily Windows XP and Windows Vista at this time), and versions of Windows are used for servers. Versions of Windows can be used for many types of applications, and Microsoft’s agreements with original equipment manufacturers (OEMs) enable it to ensure that its operating systems are used by millions of computer users.
Sun's Solaris was originally developed only for the proprietary SPARC computing platform developed by Sun Microsystems. In recent years, however, versions for Intel-compatible hardware have been developed, and Sun is pitching Solaris as an alternative server or desktop operating system. Since it is UNIX-like, it shares many similarities with Linux distributions.
Apple's Macintosh Operating System (Mac OS) is a proprietary operating system designed for PowerPC and Intel-based hardware developed by Apple. Apple's Macintosh computers have a fairly small market share, but they have become more popular over the last few years. The Mac OS is quite popular among certain communities, including filmmakers and Ruby on Rails developers. An operating system based on a minimized version of the Mac OS powers Apple's iPhone.