The Free On-line Dictionary of Computing (18 March 2015):
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.