Honestly, when you hear the word Linux do you automatically think of the kernel by itself or do you think of an entire OS?
The kernel, of course. GNU/Linux is the most popular OS based on Linux, but there are other OSes based on it (usually for embedded systems, where the GNU userspace is too large).
Silly of me to have asked the question in the first place then? I'm aware of many differences between distributions. Still, I wouldn't hasten to claim that the kernel is the only similarity between many or even most distros.
Most distributions are GNU/Linux -- ie, they use the GNU userland tools (gcc, bash, coreutils, sometimes Gnome) and the Linux kernel. GNU/Linux is the most popular OS based on the Linux kernel, but there are others. Android and Chrome are two examples of non-GNU operating systems based on Linux.
Really? Do you think the average Linux user thinks that? Compare kernel.org with linux.org. Or the Wikipedia entries on each, where this topic has been done to death.
Personally I disagree: I think 'Linux' is actually multiple, fragmented OSs, in the same way that 'BSD' is.
The kernel, of course. GNU/Linux is the most popular OS based on Linux, but there are other OSes based on it (usually for embedded systems, where the GNU userspace is too large).