Resource Fork
Pre-OS X and some OS X Macintosh files have two forks: the data fork, and the resource fork, which is subdivided into resources. Many file formats use the resource fork to store data, stored in Macintosh resource file format.
A resource has a four-character type code, an ID, and data. The type code is generally unique to the type of data stored within, but this may not be the case with application-specific formats.
Resource forks are usually invisible to the end-user on a Mac platform, but when Mac files are placed in a medium where users of other operating systems can see them (e.g., Windows), they are often visible as an additional subdirectory called resource.frk, or else a name with an underscore (_) as its first character (e.g., _MACOSX for OS X files) containing subdirectories and files that parallel the structure of the files they are associated with.
Later (Unix-based) Mac systems used AppleSingle and AppleDouble formats instead of the "classic" resource fork format when using filesystems not supporting direct use of resource forks. This usage continues in some OS X filesystems.
References
- Resource fork (Wikipedia)
- More Macintosh Toolbox (format is described on page 1-121 to 1-125)
- KSFL reads and writes Macintosh resource files
- Description of the file format from the KSFL wiki
