Resource Fork

On MacOsClassic, files consisted of a data fork, a resource fork, or both. The data fork was unstructured data, addressed by byte offset, like files on other operating systems. The resource fork was structured in a manner similar to IFF (which was popularized on the Amiga), except not with quite as much versatility: resources can't be nested the way IFF hunks can.

The trouble is that you tend to lose the ResourceFork and other metadata when transferring files between operating systems... Since the NextStep -> MacOsx transition the resource fork is being phased out in favor of bundles, specially structured directories in which resources are stored as standalone files.