Let us now consider some rough categorisations of operating systems.
the simplest kind of OS might be known as a single-user, single-tasking system. Rather of out of date now, but....
eg: CP/M, MS-DOS
Not very common nowadays, though many of the personal computers of the 1980's were single-user, single-tasking systems (mostly through not having enough memory to run multiple programs)--- if you wanted to edit a document, you'd load in the word processing software. If you wanted to play a game, you'd quit the word processor, and load up Jet Set Willy or something.
Up from that we might have single-user, multitasking system, like Windows95, with which you are undoubtedly familiar, and the MacOS, and the AmigaOS.
(There is a difference between preemptive multitasking, where processes need not be aware of other tasks using the CPU, and cooperative multitasking, where threads voluntarily give up the CPU, usually while waiting for a certain event to occur.)
No need for user authentication or maintaining multiple user accounts.
However: even on 'single-user' machines, we are seeing increasing levels of security and robustness--look at Windows'95. OSs for home computers, even, are beginning to allow for different users, even if it's only Mum, Dad and the kids, and we are beginning to see user authentication and file protection appear on most mainstream OSs. Ultimately the only machines which are regarded as 'single user' may be intimate computers like personal organisers, wallet computers and personal audio equipment.
e.g.: UNIX, WindowsNT, BeOS
Since multiple users, often must ensure reasonable response times for users.
One important property of an operating system is that it be 'scalable'-- the OS should be able to cope well in a variety of environments from palmtop machines to laptops to network computers to file servers and supercomputers. Nowadays a 'general purpose' operating system is usually designed to be scalable in this way.
I also want to just mention distributed Operating Systems, since they Are The Future...
... but you will look at these in more detail in the Operating Systems II class.
e.g.: Inferno, Clouds
A distributed Operating System is one which coordinates many computers on a network, usually serving many users. A distributed operating system should make the network 'transparent', so that users do not care where there programs are being run, or where there files are stored, and so that network resources (like printers) are as easy to access as local resources.
We don't really get into distributed OSs in this class, but you should know that they exist.
We should also mention real-time operating systems
e.g.: ...TRON
Used in situations where there are hard deadline to be met, often in process control situations (like controlling factories, washing machines and F15s). ...there's a 4th year class, real time systems, which looks in a bit more detail at the problems associated with real-time and reliability requirements.
Some real-time features making their way into mainstream OSs, (especially those with poor multitasking, like Windows and MacOS), to allow reliable multimedia display (avoid skipped frames or jumpy audio), and to control hardware devices.