Operating system
The operating system is generally the biggest piece of software running on a computer.
Operating systems utilize a kernel, which contains drivers to drive the pieces of the system, such as the CPU, video card (GPU), the sound processor, NIC etc.
The rest of the OS often composes of tools that the user interfaces with.
Some operating systems, like Mac OS and Windows are proprietary. Mac OS is only permitted to run on a very small selection of hardware. Windows is permitted to run on IBM compatible PC's, but it runs on x86, AMD64 (alternately x86-64) and certain ARM (relying heavily on an x86 interpreter/JIT) CPU's. Computer hardware/peripheral manufacturers typically assume that Windows is the only OS that exists and produce a driver for Windows only.
There are a number of free (as in freedom) OS's, but the one with the greatest compatibility is GNU/Linux, although systemd/Linux and busybox/Linux are also quite popular. Linux (a kernel) was originally developed on a 80386 CPU (on an IBM compatible PC), but soon gained support for (newer) x86, AMD64, ARM, alpha, PPC and sparc CPU's (just to name a few). Linux is often used in conjunction with the GNU OS to make GNU/Linux. While Linux comes with lots of drivers and generally has excellent support of hardware based around x86 and AMD64 CPU's, a lot of computer hardware manufactures tend to only make a Windows driver (plus usually don't provide any hardware documentation), meaning that someone needs to reverse engineer the hardware and write a driver before it can be used on GNU/Linux. For things like GPU's and Wi-Fi cards, manufactures tend to know that GNU/Linux exists, but still don't provide a driver or hardware documentation - they prefer to supply a binary blob instead. Binary blobs usually work, but as they are proprietary, they can't be really be debugged or improved. If a Wi-Fi card or GPU doesn't work on GNU/Linux, 99% of the time it's the manufacturers fault.
In the days where computers were widely available, but prior to more modern operating systems, the OS was operated from a simple command line (e.g. MS-DOS). Not long afterwards, graphical operating systems (i.e. Windows) were introduced, allowing normies to use computers and also improving the ease of performing graphical based tasks. The command line is widely believed to be a thing of the past, and GNU/Linux can be used without even seeing a CLI, but really, CLI's and GUI's have certain uses that either excels at.
Windows is widely believed to be the most popular OS, but most likely it is actually MINIX - as that's what Intel uses to power their ME backdoor.
