OLE Compound Files
Compound file is a file-based implementation of OLE Structured Storage that is a tree-like structure made of two types of objects: streams and storages. Structured Storage is very similar to the file system - there are streams, which contain data (like files), and storages, which don't have any associated data, but can contain streams and other storages (like directories). You can consider a compound file as a file system in a file.
The most common example of compound files is a Microsoft Office files. The structure of a typical Microsoft Word file may look like this:
The file on the screenshot consists of the root storage, one empty substorage, and five data streams.
Of course, editing MS Office files with a hex editor is not a very common task, however if you are a developer using Structured Storage in your program, you will find this feature indispensable.