File systems are an integral part of any operating systems with the capacity for long term storage. There are two distinct parts of a file system, the mechanism for storing files and the directory structure into which they are organised. In modern operating systems where it is possible for several user to access the same files simultaneously it has also become necessary for such features as access control and different forms of file protection to be implemented.
A file is a collection of binary data. A file could represent a program, a document or in some cases part of the file system itself. In modern computing it is quite common for their to be several different storage devices attached to the same computer. A common data structure such as a file system allows the computer to access many different storage devices in the same way, for example, when you look at the contents of a hard drive or a cd you view it through the same interface even though they are completely different mediums with data mapped on them in completely different ways. Read more at OCModShop.