Source : Free On-Line Dictionary of Computing

Macintosh file system
     
         A file on the {Macintosh} consists of two parts,
        called forks.  The "data fork" contains the data which would
        normally be stored in the file on other operating systems.
        The "resource fork" contains a collection of arbitrary
        attribute/value pairs, including program segments, {icon}
        {bitmaps}, and parametric values.  Yet more information
        regarding Macintosh files is stored by the {Finder} in a
        hidden file, called the "Desktop Database".
     
        Because of the complications in storing different parts of a
        Macintosh file in non-Macintosh file systems that only handle
        consecutive data in one part, it is common to only send the
        Data fork or to convert the Macintosh file into some other
        format before transferring it.
     
        (1996-03-03)