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$System is a string describing the type of computer system on which Mathematica is being run.
$System typically consists of words separated by spaces.
Typical values are "Microsoft
Windows", "Macintosh", "SPARC" and "Silicon
$SystemID provides a more succinct version of the same information.
See the Mathematica book: Section 2.13.5.
See also: $SystemID, $Version, $MachineType, $ProcessorType, $OperatingSystem.