Quiz: Remember the definition of mal de mer?
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.