What precisely constitutes an OperatingSystem is a widely and wildly debated topic. Usually it's considered to mean a Kernel and a minimal set of UserSpace applications to get the machine to Boot into a state capable of running user programs. It is commonly executed immediately after the BootStrap part of a PersonalComputer's (not sure about mainframes, but probably the case for them as well) startup sequence.
The most common variants of OperatingSystems in general use are:
An entire class unto themselves are the UNIX and Unix-like OperatingSystems:
An original Unix, such as
See Category:OperatingSystem for another list and help with all the acronyms.
Many others are undoubtly available to the persistent user, or someone may have been brave enough to have written their own2?.
1? QDOS is usually referenced in one of two ways. The first as "Quick Disk Operating System", the other as the correct "Quick and Dirty Operating System". 2? Writing your own operating system can be a lengthy and complicated process which will involve some form of AssemblyLanguage programming. However it gives a great sense of satisfaction at having completed such a difficult task.
138 pages link to OperatingSystem: