http://arstechnica.com/apple/2005/04/macosx-10-4/7/#extended...
Today, resource forks are actually exposed through the extended attributes interface:
http://arstechnica.com/apple/2013/10/os-x-10-9/9/#tags-imple...
You can see the source code for all this in Apple's Darwin open source repository. Example: http://opensource.apple.com/source/xnu/xnu-2782.40.9/bsd/hfs...
Please read this, written by the creator of resource forks, to learn why they exist:
http://www.folklore.org/StoryView.py?story=The_Grand_Unified...