An operating system (OS) is basically a program (usually: a set of programs) that mediates between human users and the computer, and that can do most of the things most programs that may run on the computer need: storing and retrieving from disks or other media; handling user input; drawing to the screen; and more.
I have some experience with six kinds of OS, namely in historical order CP/M, DOS, Apple, Windows, Mac, Linux. All of these OSs had versions, often many, of which generally the later ones were more capable than the earlier ones, and for most there was a rather intimate relationship with a certain kind of computer hardware.
There are more OSs, and there have been considerably more, but currently most computer users work with Windows, Mac or Linux.
CP/M I knew from an Osborne computer produced in 1982 or 1983. It was an early OS for what at the time was called "home computing", and is supposed to have been rather well-designed. I can't say much about it, except that it did its job on the Osborne quite well.
DOS - acronymic speech for "Disk Operating System" - was (rapidly) designed for the IBM PC and similar computers and was marketed very successfully by a small software company called Microsoft, that had bought it from its developer.
Both CP/M and DOS were 'character based' in the sense that they primarily dealt with the screen as if it was made of series of rows of places on which one could place one character. The reason was that personal computers around, say, 1985 did not have enough processing speed and memory to do much fancy processing with a screen made up of pixels.
Apple - I use this name, somewhat imprecisely, for rectangular boxes with small graphical black and white screens - I learned to use in the university in the late eighties. They were in many ways, being graphical, more pleasant to look at and work with than what DOS could do on a PC, but they were also more expensive.
Windows I knew first working under DOS, where it did produce a graphical environment like that of Apple, except that it was much less powerful, quite slow, and prone to crashes, for which reason I never did much with it on PCs running DOS.
Mac is the name I use for the follow-ups of the Apple I described above. I have some experience with this, but not much.
Linux I have tried quite a few times over the last 8 years. For various reasons - open source, no Microsoft - it is better than Windows, but I have had little luck with it and for the moment and the last years it is less fit for my purposes (and health: I don't have the health even if I had the inclinations to puzzle out the arcana of Linux).
I am in principle much in favour of Linux, but from what I've seen so far from it the developers seem not to have been able as yet to lift it beyond an OS for techies.