I’d just like to interject for a moment. What you’re refering to as Linux, is in fact, GNU/Linux, or as I’ve recently taken to calling it, GNU plus Linux. Linux is not an operating system unto itself, but rather another free component of a fully functioning GNU system made useful by the GNU corelibs, shell utilities and vital system components comprising a full OS as defined by POSIX.

Many computer users run a modified version of the GNU system every day, without realizing it. Through a peculiar turn of events, the version of GNU which is widely used today is often called Linux, and many of its users are not aware that it is basically the GNU system, developed by the GNU Project.

There really is a Linux, and these people are using it, but it is just a part of the system they use. Linux is the kernel: the program in the system that allocates the machine’s resources to the other programs that you run. The kernel is an essential part of an operating system, but useless by itself; it can only function in the context of a complete operating system. Linux is normally used in combination with the GNU operating system: the whole system is basically GNU with Linux added, or GNU/Linux. All the so-called Linux distributions are really distributions of GNU/Linux!

Linux is the kernel; that is, the core of the operating system, which handles memory, hardware, inputs… Every OS has one. Windows’ is called NT, macOS’ is called Darwin.

You don’t use the kernel, you use the OS. Linux is special because there several operating systems based on Linux; they are called distributions, and they are what you want to use.

You can create your own Linux distribution, by bundling various software packages with the kernel, in a way that caters to specific needs or follows a philosophy (for example, Linux Mint is a distribution focused on ease of use, Archlinux on minimalism…). This is not possible with Windows because their kernel is not licensed under terms which would allow this. It is with Darwin, but unusual.

You can also create your own kernel but this is extremely hard. Getting the computer to boot is easy enough (relatively speaking), but getting programs to run and things to display is much harder. Getting your custom kernel to a usable state is orders of magnitude harder, as it needs to work on and communicate with modern hardware and networks. Linux took more than 30 years of development by thousands of developers (some of them highly talented in a very narrow field of computing) to get to the level it’s at.

Linux is a combination of the Linux kernel (some really low level software that tells the OS how to communicate with your hardware, not something most users should worry about) and software from GNU (some Linux users call it GNU/Linux for this reason). This is then used as the base for distros, which are basically just that plus a package manager (how you install apps) and usually a desktop environment and default apps as well.

Theoretically yes, you could create an operating system and name it Steve, but it wouldn’t be Linux unless it had the Linux kernel and GNU software.

Edit: as others have pointed out, yes, you can technically have linux distros without GNU, but GNU-based Linux distros are by far the most popular option, so you’re highly unlikely to find a distro without GNU unless you’re hunting for one.

Linus.

The kernel. That’s the part that is literally called “Linux”, i.e. the thing that has all the drivers for your hardware and regulates of multiple applications can run at once on the same machine.

On top of that comes the GNU userland (a project started before Linux to build a free Unix), this gets a bit more fuzzy, this includes things like glibc, all the basic tools, shell, ls, tar, rm, gcc, … It’s fuzzy because it’s not strictly required to run Linux. Android for example runs the Linux kernel, but has a completely different userland than a typical Linux installation, much of which use the GNU tools. And than you have things like systemd, X11, KDE, Gnome, … that aren’t really GNU userland tools, but rather important to what most people would expect from a “Linux desktop”.

“Linux” has two meanings. One of them is the kernel itself; another is a collection of operating systems, that Stallman would call “GNU plus Linux” instead.

The later involves two factors. A “hard” one is the presence of the Linux kernel; but there’s also two softer and fuzzier ones:

• the operating system behaves like other OSes that the user calls “Linux”. For example you’re expected to have a /home/username, you can install a different DE/WM if you want, this kind of stuff.
• the OS is open in letter and in spirit. This is ideological but ideology is damn important when dealing with Linux.

A good example of both is ChromeOS: people don’t usually call it “Linux”, even if it uses the Linux kernel. It’s simply too atypical in behaviour, and ideologically too distant from the open source movement.