Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
"Linux is just a kernel." It's a commonly heard refrain amongst the arguments put forward by BSD Unix aficionados, and it is a true statement, but it is all too often abused to try to make a fallacious point.
FreeBSD, NetBSD, and OpenBSD are complete core operating system projects. Each of these projects has integral to it the concept of a "base system", which consists of the kernel, basic userland, et cetera. The Linux project itself is only the kernel, by contrast; to make a complete operating system, you need to add a bunch of stuff from outside the Linux project. Many people and organizations have done so, resulting in a plethora of what are known as "Linux distributions". These distributions, or "distros", of Linux vary significantly from one case to the next.
A typical BSD-oriented list of differences between a BSD Unix OS and Linux will point out that "Linux is just a kernel," with a wide variety of distributions being built on top of it, then go on to explain how (to quote the FreeBSD and Linux post at RootBSD):