Linux is a kernal, Unix is a type of OS. Unix is a registered name so Linux cannot be described as a Unix OS however you will hear Linux described as a Unix like system. One thing to bear in mind is that Linux is NOT an OS it is just the kernal the GNU (recursive acronym for GNU's Not Unix) is what people often confuse as being 'Linux' this is however the command line and tools that interact with the Linux Kernal.
Unlike Unix, Linux isnt made by any company or single group of people. Linus started the Linux Kernel, and ultimatly has the end say in what happens to Linux, however it is programmed by a global network of people who simply code for the fun of it.
Linux is just a kernel. A kernel is the first thing loaded when you boot, it controll's all hardware, and contains all of your drivers.
GNU is software, Like the C, C++, Assembly and other compilers, also libraries, and other base things that run on top of the kernel.