Linux is kernel, that is the heart of all Linux distros.
Linux an be downloaded from
http://kernel.org/
Linux itself is not an operating system.
Linux distros like RedHat, Debian, etc... are build by combining many user land software (GNU) with linux kernel. Since Linux is used as kernel, most of these Linux Distros are generally knows as Linux OS.
You can build your own Linux distro if you want, find more details at
http://www.linuxfromscratch.org/
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