Linux is Unix, AT&T invented Unix at Bell Labs. End of Story.
Um... no. Linux is not UNIX, but it is a POSIX-compliant operating system (like UNIX), and most of the standard UNIX tools have been ported to Linux, which makes the systems look similar to an end user. But they aren't the same.
Windows 95 contained more actual UNIX code (TCP stack) than Linux does...
http://www.linuxinsider.com/perl/story/32719.html
History may give us a place to start. Everybody knows Unix started at what was then AT&T Bell Labs and went through two enormous growth phases, the first at Berkeley in the early 1980s and the second at Sun 10 years later, but surprisingly few people know where Linux came from.
The current kernel is fundamentally the work of Linus Torvalds and people he recruited into the kernel development and maintenance process, but the kernel he started with -- Andrew S. Tanenbaum's Minix -- was unambiguously described by its author as "an open source Unix clone."
So Linux is actually a work based on a clone of UNIX, using GNU software (which is *not UNIX*)