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.
The format is described in detail on pages 1-121 to 1-125 of Inside Macintosh: More Macintosh Toolbox.