Hierarchical Directory Structure

While the file systems used in some very old operating systems did not include the concept of directories, all commonly-used file systems today include this feature. Directories are themselves usually implemented as files, meaning that no special utilities are required to maintain them.
Furthermore, because directories are themselves files, and directories contain files, directories can therefore contain other directories, making a multi-level directory hierarchy possible. This is a powerful concept with which all system administrators should be thoroughly familiar. Using multi-level directory hierarchies can make file management much easer for you and for your users.