Most [operating systems]? provide a file system, which is usually presented to the user graphically using the metaphor of folders containing documents and other files. The hierarchy is presented either as a tree or as nested folders.
Most file system sits on top of an underlying storage system that manipulates an array of fixed-size blocks. The file system software is responsible for organizing these blocks into files and directories, and keeping track of which blocks are not being used. Usually the underlying storage system is non-volatile storage such as a hard drive or [flash memory]?.
File systems include: