Linux is a hugely popular OS outside of the desktop market. Android (the mobile operating system acquired by Google) is based on it, and it runs on most of the world's smartphones. Sometimes it is the main desktop option, if using a computer with specialized hardware, such as the Raspberry Pi. Linux is very popular on embedded devices, in part because it is lightweight and usually much less demanding on resources than a typical Windows or macOS installation.
Linux is also the most popular operating system for web servers, which is what interests us most here. It could be argued that Linux is the most popular operating system on the planet, although many users won't interact with it directly.
Note
Strictly speaking, Linux only refers to the kernel of the OS. The equivalent part of Windows would be the NT kernel. Many different distributions are built using the Linux kernel at their core, and they bundle it up with many other (often GNU) applications. You can even roll your own stripped...