This document discusses different file directory structures used in operating systems. It describes single-level, two-level, tree-structured, and general graph directory structures. Single-level directories have each user get their own directory space with unique file names. Two-level directories add the concept of a root directory and allow specifying the directory being accessed. Tree-structured directories allow files to be accessed in multiple places using symbolic links. General graph directories allow cycles but can cause problems for search algorithms and garbage collection.