[chipotle_coyote]

Basically, the "resource fork" in MacOS was... well, Wikipedia actually puts it pretty well: "a section of a file used to store structured data along with the unstructured data stored within the data fork." 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/value store -- alongside a file.

For a practical example of the metadata, albeit not one from MacOS: BeOS had a similar concept of file attributes as key/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.

[krylon]

That sounds kind of awesome!

I never got to use BeOS, and Haiku seems to be stuck perpetually in the "almost there"-stage (Haven't been paying attention in the last couple of years, though). But this helps me understand why people worked and work so doggedly on reviving BeOS.

[mtone]

Howerver, in NTFS at least, attributes/metadata is a pretty flexible database-like storage, yet are not the same thing as alternate streams (a.k.a. resource forks) which are also supported.