Macintosh resource file
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.
See Resource Fork for general information about Macintosh resource forks.
A resource has a four-character type code, an ID number, an optional name, 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.