A file is a sequence of bytes. No structure is imposed on a file by the system, and no meaning is attached to its content-- that is, the meaning of the bytes depends solely on the programs that interpret the file. The Linux file system is structured as a tree. The leafs of the tree are ordinary files. The internal nodes of the tree are called directories. A directory is a special file that contains pointers to other files and directories. A subdirectory is a child of another directory.