DirectoryName["name"] extracts the directory name from the specification for a file.
DirectoryName works differently on different computer systems.
DirectoryName["directory"] is normally equivalent to ParentDirectory["directory"].
DirectoryName["name", n] applies DirectoryName n times to name.
DirectoryName yields output appropriate for use in SetDirectory and ToFileName.
If name contains no directory specification, DirectoryName["name"] returns "".
See The Mathematica Book: Section 2.11.4.
See also: $Input, Directory.