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.
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.