This is the instructions that the computer uses to tell itself how it "operates". It's the answer to "Who am I and what can I do?"

Some common operating systems are DOS, various versions of Windows, OS/2, UNIX, LINUX, System 7. These all behave in very different ways and have different hardware requirements. So they won't all run on all machines.

Only the parts of the operating system that are currently being used will be loaded into Main Memory.