Basically, the &quot;resource fork&quot; in MacOS was... well, Wikipedia actually puts it pretty well: &quot;a section of a file used to store structured data along with the unstructured data stored within the data fork.&quot; It was frequently used to store things like images and icons (and fonts), but could also be used to store metadata. Think of it as the equivalent of having a document database -- a key&#x2F;value store -- alongside a file.<p>For a practical example of the metadata, albeit not one from MacOS: BeOS had a similar concept of file attributes as key&#x2F;value pairs, and some MP3 players would store ID3 tags in files. In a directory window, you could list any custom attributes as columns. When I was a BeOS user, that made it trivially easy to use the file system itself as my answer to iTunes: windows would show artist, album, song title, track number, and playing times (or whatever I wanted), and because you could search on custom attributes and then save those searches as virtual folders, you could easily set up the equivalent of smart playlists.