An operating system (OS) is a suite of software that manages the services of a computer. A computer user would have to be an expert in hardware programming in order to use the computer without the operating system, because it is the operating system that interprets every keystroke, click and mouse movement into actions in the processor of the computer.
An operating system comprises many files of varying complexity. The Windows operating system sits on top of an interpreter which in turn relies on underlying programs that talk to the registers in the processor. At the lower level, the operating system guards access to resources, like the processor or network card. Higher up, it translates between the requests of applications running on the computer and the physical resources that fulfil those requests. Thus, the OS provides a common platform for all applications.
The operating system has to manage the availability of the resources of the computer and make sure that all programs it serves have equal access to hardware. The operating system shares access “simultaneously” so that it seems that several programs are running through the processor at the same time. In fact, the operating system breaks down the functions needed to satisfy a request and maintains a queue of tasks waiting to be served. The rapid switching of smaller tasks means that, in reality, each program has a flash of activity in the processor for one small step of its needs, and then has to wait while all other programs take a turn.
The process of switching tasks is so fast that the delay in steps of each running program is hardly noticeable. However, if the user starts too many tasks, then the lengthening of the queue causes the cycle between task to be overwhelmed and all tasks will slow down. This poor performance is not usually the fault of the operating system, but indicates that the computer owner needs a better computer for the performance he requires -- poor performance is usually a hardware issue.
The operating system is also responsible for the look and feel of a computer. Although most operating systems allow users to customise themes and change the background picture, anyone switching between computers running Windows or Mac OS will instantly recognise the difference between the two. Computer users' loyalty to a particular computer type is, in fact, a preference for a particular operating system’s presentation.
The “virtual” resources of a computer also need organisation, and this too is the responsibility of the operating system. These virtual resources include programs and files. It is no accident that files on a computer fall into a structure of folders. This is the operating system's method of organising resources. As well as organising those resources, the operating system allows access to them. Both users and programs need to access files and the method of requesting them differs whether the request comes from a user or a program. The user's access to files is usually mediated by a file managing program which is a key part of the operating system.