UNIX & GNU/Linux

UNIX is an operating systems (OS) that has inspired others.
Indeed a lot of OS are children of UNIX, some are free.

Linux is a free kernel for many OS.
This kernel is present in all "GNU/Linux" distributions.
Linux is not child of UNIX, although it is inspired.

GNU is an acronym for Gnu is Not Unix.
It's a set of free tools and software.
It is almost a complete OS but his kernel is not completely optimized.
So it adopted Linux as its kernel.

UNIX & GNU/Linux adhered to the POSIX computing standard, establishing a great interoperability with all POSIX operating systems.