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”.