As for the differences between Unix and Linux... Basically, different kernels. IIRC, Linux could end up an official flavor of Unix. An OS only need conform to the POSIX standard in order to be called Unix.
(If I'm in error about POSIX=Unix, please fellow A.org users, correct me)
Hate to correct such an interesting response but you are right in your concern that POSIX might not == UNIX.
You only have to be POSIX compliant to be... POSIX compliant.
Linux has been POSIX compliant but not certified for a while now although that might have changed.
Linux is a firm member of the uncertified "UNIX" family.
The best place to ask is comp.unix.questions, and here is one that was prepared earlier:
Subject: What makes an OS a "UNIX" OS?
I had a long discussion with some co-workers yesterday about what is and
isn't UNIX.
We argued whether or not Linux was "UNIX". In AEleen Frisch's book
"Essential System Administration, 2nd Edition" from O'Reilly, AEleen
states that Linux is a "..UNIX like" operating system. But why? Why is
it "UNIX Like" and the other OSs (ie Solaris, AIX, HP/UX, etc...)
mentioned in the book are not labeled as such. They are simply "UNIX".
Also, a few of my co-workers wanted to argue whether or not a *BSD
system was UNIX. If I'm not mistaken, BSD was a licensed copy of AT&T's
original UNIX OS for them to do with as they please.
Most of the current "UNIX" or "UNIX-like" OSes today are simply a meld
of SysV or BSD.
So what gives? Personally i just throw them all under the heading of
UNIX. But, then I ask myself 'is Mac OS X a UNIX?' and I find myself
saying 'well, its UNIX-like', heh =)
What must an OS have to be called UNIX?
... but the real answer is certification.
The strict answer is: Only systems that the Open Group
has certified as being UNIX(tm) are UNIX(tm). See
"http://www.unix-systems.org/what_is_unix.html". Solaris,
Tru64, AIX etc. (particular hardware/software combinations)
falls into this category. History doesn't enter into it.
So UNIX == UNIX && POSIX == POSIX but POSIX => UNIX and ! UNIX => POSIX
