Macintosh resource file
From Just Solve the File Format Problem
The Macintosh resource file format was used by the Resource Manager in Mac OS Classic for storing resources used by Mac OS Classic applications as well as file associations and executable code. Traditionally, this format is used in the Resource Fork of a file on the HFS and HFS+ file systems; however, when resource files are still used in Mac OS X applications, the same format is used in a file with the extension .rsrc.
References
- Resource fork (Wikipedia)
- More Macintosh Toolbox
- KSFL reads and writes Macintosh resource files
- Description of the file format from the KSFL wiki