This should be in the *nix forum.

Well to explain it simple.

Unix is an operating system just as Windows. Bu mostly used on servers / mainframes. And very expensive. There also are free *nix clones out there. you have the bsd part (freebsd, openbsd and netbsd, the most common), Plan9 (pretty unknown but it's there somewhere ) and offcourse pretty known, linux (slackware, ...)

So if you want to learn about *nix get yourself a few books about it and i'd suggest getting some linux flavor (linux distro) (cause linux still is the most easy form of *nix to start with) and start experimenting.
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