Single-user, multi-user, multi-tasking and distributed operating systems
Single-user operating systems
Some operating systems are designed for single users. DOS, Windows and Linux are good examples. They allow a single user to access any particular computer at any particular time.
Multi-user operating systems
Sometimes, you need to have a database of information stored centrally but it needs to be accessed by lots of people. For example, you might have a database of products and their prices on a computer in a supermarket. Lots of terminals need to access the database. Each terminal is given a very short amount of computer time before the next terminal is serviced. This happens so quickly that each terminal thinks it is the only one with access to the database! This kind of system is known as a multi-user operating system.
Multi-tasking operating systems
Some operating systems allow you to open more than one application at a time. You could have open a word processor and a chat room, for example, and have an MP3 player playing in the background. Many single-user operating systems also allow multi-tasking. Windows does, for example, but DOS (one of the first operating systems for computers) doesn't. DOS is a single-user single-task operating system.
Distributed operating systems
This is where the software that makes up the operating system is divided up and distributed amongst computers that are physically apart from each other although they are connected across a network. Each computer is therefore responsible for running its share of the whole operating system. In addition, each computer has some software to control how it communicates, interacts and synchronises its jobs with the other computers on the networked system. The responsibility for running the operating system is now a collective responsibility rather than relying on one main computer.