When seen from the outside, a file system is a collection of files and directories, plus operations on them. Files can be read and written, directories can be created and destroyed, and files can be moved from directory to directory. Most modern file systems support a hierarchical directory system in which directories may have subdirectories and these may have subsubdirectories ad infinitum.

When seen from the inside, a file system looks quite different. The file system designers have to be concerned with how storage is allocated and how the system keeps track of which block goes with which file. Possibilities include contiguous files, linked lists, file-allocation tables, and i-nodes. Different systems have different directory structures. Attributes can go in the directories or somewhere else (e.g., an i-node). Disk space can be managed using free lists or bitmaps. File-system reliability is enhanced by making incremental dumps and by having a program that can repair sick file systems. File-system performance is important and can be enhanced in several ways, including caching, read ahead, and carefully placing the blocks of a file close to each other. Log-structured file systems also improve performance by doing writes in large units.

Examples of file systems include ISO 9660, MS-DOS, and UNIX. These differ in many ways, including how they keep track of which blocks go with which file, directory structure, and management of free disk space.