Unix itself is a broad family of operating systems some open source, most not (eg Tru64, Solaris, the BSDs (nearly all free + open source), etc) which have been made by various companies (Sun, IBM etc) over the years.
The GNU (GNU is Not Unix) collection of programs was designed to be free open source tools that behave in similar ways to their proptietry unix counterparts.
Then you have the Linux Kernel itself which is also open source. The kernel is also not Unix but is designed to behave in a similar manner (it was originally based on Minix)
When you combine the two you get GNU/Linux (which is commonly just reffered to as 'Linux'), which is not actually unix but it is a unix-like operating system.
Really though, Linux Behaves like unix, looks like unix, smells like unix and (probably) tastes like unix, the difference is just a difference in code-base.
On the topic of Unix/Linux.
The GNU (GNU is Not Unix) collection of programs was designed to be free open source tools that behave in similar ways to their proptietry unix counterparts.
Then you have the Linux Kernel itself which is also open source. The kernel is also not Unix but is designed to behave in a similar manner (it was originally based on Minix)
When you combine the two you get GNU/Linux (which is commonly just reffered to as 'Linux'), which is not actually unix but it is a unix-like operating system.
Really though, Linux Behaves like unix, looks like unix, smells like unix and (probably) tastes like unix, the difference is just a difference in code-base.