Kassem wroteWelcome to the site, hopefully you'll enjoy your stay :)
I don't want to sound stupid but what's a *nix?
UNIX is an operating system developed in the late 60s. It is still today one of the most popular and powerful OS around. It was originally developed by AT&T (who incidentally invented the C language for that purpose). However, due to permissive licenses, a lot of different people developed their own version of Unix, most notably the University of California, Berkley. Today there are literally dozens of Unix variations like Linux, GNU (incomplete), FreeBSD, OpenBSD, NetBSD, HP-UX (owned by HP), Solaris (originally owned by Sun, now Oracle) and ... Mac OS X.
All these variations have some difference in the tools they use, but have also a lot in common as they all follow (more or less accurately) a set of rules (standards) called POSIX.
The term UNIX is still licensed to AT&T for their own OS, people will define other flavors by *nix.
(*nix is just an easy way of saying Posix compliant OS.