The file system structure is the most basic level of organization in an operating system. Almost all of the ways an operating system interacts with its users, applications, and security model are dependent on how the operating system organizes files on storage devices. Providing a common file system structure ensures users and programs can access and write files.

File systems break files down into two logical categories:

Shareable files can be accessed locally and by remote hosts; unsharable files are only available locally. Variable files, such as documents, can be changed at any time; static files, such as binaries, do not change without an action from the system administrator.

Categorizing files in this manner helps correlate the function of each file with the permissions assigned to the directories which hold them. How the operating system and its users interact with a file determines the directory in which it is placed, whether that directory is mounted with read-only or read/write permissions, and the level of of access each user has to that file. The top level of this organization is crucial; access to the underlying directories can be restricted, otherwise security problems could arise if, from the top level down, access rules do not adhere to a rigid structure.