The directories are structures in which some kind of information is being stored. The simplest way to put it – directories are folders and subdirectories are the subfolders.
Directories are used for the organization of files, which are under a hierarchical structure. In most manuals they are described as an inverted tree. The folder that sits the highest in the hierarchy is called root directory, and all the ones that are below it are called subdirectories. The directory that sits above a subdirectory is called parent directory of that subdirectory.
Each and every operating system has directories. In Linux based operating systems there are two types of directories – system directories and user directories.