$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", "Power Macintosh", "Linux" and "Solaris".
$SystemID provides a more succinct version of the same information.
See Section 2.14.5.
See also: $SystemID, $Version, $MachineType, $ProcessorType, $OperatingSystem.
New in Version 1.