What is Linux?
Linux is Simply an Operating System. It is open source.
Linux is UNIX Clone
But if you consider Portable Operating System Interface (POSIX) standards then Linux can be considered as UNIX. To quote from Official Linux kernel README file:
Linux is a Unix clone written from scratch by Linus Torvalds with assistance from a loosely-knit team of hackers across the Net. It aims towards POSIX compliance.
However, “Open Group” do not approve of the construction “Unix-like”, and consider it misuse of their UNIX trademark.
Linux is just a Kernel
Linux is just a kernel. All Linux distributions includes GUI system + GNU utilities (such as cp, mv, ls,date, bash etc) + installation & management tools + GNU C/C++ Compilers + Editors (vi) + and various applications (such as OpenOffice, Firefox).
As I said earlier Linux is just a kernel and Linux distribution makes it complete usable Operating Systems by adding various applications. Most UNIX Operating Systems comes with A-Z programs such as editor, compilers etc. For example HP-UX or Solaris comes with A-Z programs.
Top 5 Linux Distributions Linux OS:
Redhat Enterprise Linux.
Suse Enterprise Linux.