A file is a sequence of small pieces of data. It is more important to us that, on most computer systems, the collection of files is organized in directories . Roughly speaking, a directory contains some files and some more directories. The latter are called subdirectories and may contain yet more subdirectories and files, and so on. The entire collection is collectively called a file system or a directory tree .
Figure contains a graphical sketch of a small directory tree.
The tree's root directory is TS . Each box has one of two annotations. A directory is annotated with DIR, and a file is annotated with a number, which signifies the file's size.