What is the very fundamental difference between Unix, Linux, BSD and GNU? Unix was the earliest OS, so the term 'Unix like' is understandable, since they have kernel, file system structure, most of the commands, users etc are same as Unix. Still why are they different? What made them set apart? Is it the kernel?  
What is the difference between Unix, Linux, BSD and GNU?
linux;bsd;gnu
That is a difficult question to answer. Fist Unix Like or *nix usually means POSIX. All the systems you listed are POSIX systems. POSIX is a set of standards to implement.Now for the harder questions. GNU isn't really a OS. It's more of a set of rules or philosophies that govern free software, that at the same time gave birth to a bunch of tools while trying to create an OS. So GNU tools are basically open versions of  tools that already existed but were redone to conform to principals of open software. GNU/Linux is a mesh of those tools and the Linux kernel to form a complete OS, but there are other GNUs. GNU/Hurd for example.  Unix and BSD are older implementations of  POSIX that are various levels of closed source. Unix s usually totally closed source, but there are as many flavors of Unix as there are Linux if not more. BSD is not usually considered open by some people but in truth it is a lot more open then anything else that existed. It's licencing also allowed for commercial use with far fewer restrictions as the more open licences allowed.Linux is the new comer. Strictly speaking it's just a kernel, however, in general it's thought of as a full OS when combined with GNU Tools and a bunch of other things. The main governing difference is ideals. Unix, Linux, and BSD have different ideals that they implement. They are all POSIX, and are all basically interchangeable.  They do solve some of the same problems in different ways. So other then ideals and how they chose to implement POSIX standards, there is little difference. For more info I suggest your read a brief article on the creation of GNU, OSS, Linux, BSD, and UNIX. They will be slanted towards their individual ideas, but when read though you will get a good idea of the differences. 