An operating system is a piece of software that you put on your computer that will make your computer "work". In other words, with it, you can exploit the full potential of your computer. In the late nineties, Windows and macOS (which were the only known operating systems in the public) were pretty much optimized for the bad hardware we had back then (but hey, it was still something!). But nowadays, companies think that everyone has the PC of the future and completely removed the optimizations which makes them slow. Of course, that's not the case of GNU/Linux. GNU is an operating system, and Linux is a kernel. Nowadays, Linux is very popular but also misunderstood by a lot of people : some people say it's an operating system, while it's a kernel. But still, it has a great future. Anyways, let's stop talking here and move on to the first operating system! Which is...
Windows is the most common and widely used operating system in the world. Inside, it's not really bad (it has a great design and other cool features), but in the outside it really is. It's EXTREMELY slow on low-end/old PCs, and can saturate your connection if you don't configure it properly. Microsoft (which owns Windows, pretty sure you know that) is also trying to get the hands on Linux by doing things such as adding a compatibility layer to run Linux programs on Windows (called the Windows Subsystem for Linux, which isn't even real Linux and we'll see why later), adding an actual (but experimental) package manager (called the Windows Package Manager), going mostly open-source for their apps (but not Windows of course) and probably other things in the future. But what they probably don't understand is that it doesn't work like that. You can't just take someone else's work and copyright it (at least, it seems it's what they're doing to do).
Remember when I said WSL wasn't real Linux? Well, that's simple. WSL uses a custom Linux kernel made by Microsoft (obviously) which tends to be compatible with Windows, of course. But the major problem is that it's so modified that some programs made for the original Linux kernel just won't work. So a lot of people are afraid that everyone will make their programs compatible for WSL and not for the original Linux kernel anymore, resulting in everyone using WSL, and so, Windows. But, it doesn't work like that. Like I said before, they won't trash it and make only Windows relevant. Their goal is to go open-source and exploit the Linux kernel, but it won't work either. The funniest part here is when the NSA (not Microsoft) tried to add a backdoor into the Linux kernel, but Linus Torvalds obviously said "no". (It's not about Microsoft, but I thought I would share it since it would make Windows much more "relevant".)
macOS is an operating system built around a modified Unix kernel called Darwin. It's less shittier than Windows, but still has its flaws (controlled by a big corpo called Apple, for example, which doesn't respect your privacy). I can't say anything about it since I never owned an Apple product (except an iPhone 4). It's still a bit better than Windows, but the major flaw is that Apple doesn't want you to install macOS on non-Apple hardware. Obviously, on Linux (with QEMU/KVM) you can, but not on Windows (at least for now). Now let's move on to the final "operating system"!
GNU/Linux (often referred as Linux, which we'll do just now) is an operating system (again, it's a kernel, but for our own sake we'll just call it an OS) first and still developped by Linus Torvalds. It's fully open-source and extremely customizable. Now, you may not want just run the kernel, but use a distribution. A distribution (often referred as a distro) is an actual operating system built around the Linux kernel. There are plenty of them. And when I say plenty, I mean so much. For example, on my host, I use a distro called Elementary OS, which is based on another distro called Ubuntu.
I won't say what is Ubuntu, or what is each distro, this is for you to discover.
Anyways, now that we know what an OS is and them themselves, you can now dig deeper into the subject!