What is Linux?
Linux is just a kernel. Ok, that is it, all you need to know, you can stop reading now, or you can proceed at your own peril. Linux has to be more than just a kernel, right? The answer is yes and no, yes Linux is more than a kernel, and no, Linux is not more than a kernel. Confused? Great!
What is a Kernel?
When you think kernel, you may at first think corn, police, military, or if hungry the finger-licking-good KFC, but in the computing sense, a kernel is the first program that starts when your computer boots. Well, technically, the kernel is the second program, after the BIOS, which is the software that ships with your hardware, usually the motherboard, but other devices and peripherals can have their own BIOS as well.
Quick recap, BIOS that comes with your hardware starts first, then the kernel, which is the first piece of software a user has control over. With most operating systems you do not have a choice of kernels, you run the kernel the operating system ships with, such as MacOS or Windows. For further reference please see the Wikipedia pages on BIOS and Kernel.
A computer kernel controls all aspects of the hardware, any piece of software wanting to access or use any hardware resources does such via the kernel, memory allocation, device access, CPU or GPU time, etc. The kernel is the single most important piece of software to any operating system, it is central to it all, and if/when a kernel fails, or crashes, your computer is inoperable and most times must be reset via a reboot, and in that sense, Linux is no different. Linux kernel panics, a.k.a. crashes, can happen, but are typically rarely encountered compared to other operating systems. One of many reasons that make Linux superior to others, but in the sense of the kernel and operating system, given the operating system's reliance on the kernel.
Wait, you can choose a kernel? YES!
Linux is a Kernel
Yes, Linux is just a kernel, but wait, no it is not. Linux technically is just a kernel, however, because it is core and central, most all Linux operating systems, or more correctly, Linux Distributions, a.k.a. Distros, are typically referred to simply as Linux. When asked, yes, you run Linux, and no, you are not just running a kernel. Therefore, Linux is more than just a kernel, but again, technically, Linux is just a kernel. The software that is developed and ships from kernel.org, that is really all Linux is, all that Linus Torvalds started, coded, and continues to oversee, code, and contribute to this very day.
But, Linux is more than just a kernel, when we begin talking about Linux as a operating system, again, more correctly, a Linux Distro. We have another page on what is a linux distro? that covers distros in more detail and another choosing a distro to help you decide which distro might be right for you. Distros are essentially, all the other software that you see and use when running Linux, and much of that software can be run on other kernels, Free BSD, Net BSD, and even MacOS can natively run much of the very same software. It might all seem confusing at first, but eventually, it will make more sense, and you will understand the technical differences and nuances.
Back to Linux, in some cases, such as the Android operating system, which still does run a Linux kernel, but it is so heavily modified when it comes to all other aspects, Android is typically not considered Linux, definitely not a Linux Distro, and in general, is its own operating system, running a Linux kernel; a Linux derivative operating system. That is pretty much it, of course, there is much more detail to it all, but that gives you a initial understanding of Linux, kernels, and if you read other resources Linux Distros, your next stop on your Linux journey.