Whoa, no way, man. Linux *is* the OS. The different names for it (Suse, RedHat, Debian, etc.) are just *distributions* -- it's a concept not in place with Windows and MacOS, so it's not one familiar to folks who primarily use either of those. It's the same kernel in there, the same shells, etc. Things are just in different places, and each distribution has its favored GUI. So, yeah, "the difference is more than just the aspect of the GUI" which I said before. The difference between *distros*... there aren't multiple Linuxes... there are just different distributions of it, and you can substitute "packages" for distributions, to put it in more layman's terms.
Trust me, I've been under the hood with Linux probably more than 99% of the folks who read and post here. I know what I'm talking about -- been a sysadmin for the last 8 or so years, cut my teeth on the 1.0.24 kernel and even participated in benchmarking at an early stage of development... google me and the word bogomips
(OK, enough pulling out the ruler here.)