Open an executable file one way, and you get the executable content. Open it another way, you get all the GUI things it needs to let you use it (icons, menus, dialog boxes, etc). The first is called the Data Fork, the second is called the [link|http://en.wikipedia.org/wiki/Resource_fork[Resource Fork|http://en.wikipedia....ork[Resource Fork].

The idea is a specific instance of Windows NT's [link|http://en.wikipedia.org/wiki/Fork_(filesystem)|alternate data streams]. And I believe Hans Reiser has gone on record saying this concept would be a good thing to add to Linux.

Wade.