In the earliest days of the Macintosh file system, a structure was devised whereby a file could have two pieces, the data fork and the resource fork. The data fork was a single thing, and was just for data (like the text of a TeachText / SimpleText file). The resource fork was for secondary information, and could contain many resources, accessible by category and name or number, as in a kind of miniature database (so, for example, style information in a TeachText / SimpleText file).

When Mac OS X was introduced, Apple undertook a concerted effort to deprecate the resource fork, because it wasn’t a standard Unix file system thing (in fact, a Macintosh file moved to another file system, such as Windows or true Unix, will usually lose its resource fork). As part of this effort, a new format for compiled script files was devised, where the bytecode was kept in the data fork instead of the resource fork.

Ironically, Apple has recently realized that resource forks are a good thing (because they provide a place to put file metadata) and has reversed course — they’ve modified the file system so that a file can now (starting with Tiger, Mac OS X 10.4) have any number of extra forks.