Files that maintain the hierarchical structure of the file system.

A computer's file system organises stored files not as one flat list, but as a branching, tree-like structure so that related files can be grouped together and easily located.

Directories (also called folders) are the special files that hold this structure together. A directory can contain ordinary files, and it can also contain other directories nested inside it, which is what creates the parent-child, hierarchical layout of a file system. This is why you can navigate from a root directory down through several levels of sub-directories to reach a specific file.

"Descriptors", "modifiers", and "relative files" are not standard terms for the structural containers of a file system. Descriptors typically refer to identifiers used by an operating system to track open files, not the containers that build the folder hierarchy itself.

Examination reminder: picture the file system as an upside-down tree, with the root directory at the top branching into sub-directories, each of which can hold files or further sub-directories.