This article is from the Mac communications FAQ, by Bruce L Grubb BruceG6069@aol.com with numerous contributions by others.
The older Macintosh files have two parts: a data fork and a resource
fork. Text files and GIF image files are examples of Macintosh files
that are usually stored completely in the data fork, and have an empty
(or nonexistent) resource fork. Older Mac applications, as a
counter-example, store most if not all of their information as
'resources' in the resource fork and usually have an empty data fork.
Because this two-forked organization of files isn't very common,
not only did Mac archive formats have to support them but a means to
turn the two fork Mac file into a data fork had to be developed so
that mac files could pass through non-macintosh machines (such as UNIX
boxes, or MS-DOS machines) without being damaged.
This also means that without modification non-mac archives and
encoding formats cannot be used to send mac files.