For all newbie ( like me ) dont get down if you have study so hard but you still cant understand how somputersystem works. It takes time, not just like a pop up magic, abrakadabra....bam...and you can get all the system in one night.
Well basicly, computer system has a few operating sistem. First, from wikipedia operating system is the software that manages the sharing of the resources of a computer. I got nice diagram from nice site that maybe you could make it as a refferences to learn.
Operating systems can be classified as follows:

* multi-user : Allows two or more users to run programs at the same time. Some operating systems permit hundreds or even thousands of concurrent users.
* multiprocessing : Supports running a program on more than one CPU.
* multitasking : Allows more than one program to run concurrently.
* multithreading : Allows different parts of a single program to run concurrently.
* real time: Responds to input instantly. General-purpose operating systems, such as DOS and UNIX, are not real-time.

Operating systems provide a software platform on top of which other programs, called application programs, can run. The application programs must be written to run on top of a particular operating system. Your choice of operating system, therefore, determines to a great extent the applications you can run. For PCs, the most popular operating systems are DOS, OS/2, and Windows, but others are available, such as Linux.

As a user, you normally interact with the operating system through a set of commands. For example, the DOS operating system contains commands such as COPY and RENAME for copying files and changing the names of files, respectively. The commands are accepted and executed by a part of the operating system called the command processor or command line interpreter. Graphical user interfaces allow you to enter commands by pointing and clicking at objects that appear on the screen.

Between windows and linux have different basic command and system, its a choice where linux is free and windows expensive ( i guess ). And each of them have their own overplus. But if you are an operating system maniacs you will learn all of the operating system, cause it is so interesting to be part of a system research.