Also, in many GUIs, a folder icon usually symbolizes something that has other icons/items/objects inside. In most GUIs I've seen, the folders looks like a cardboard folders with some papers inside.
Usually, in graphical file managers, directories are represented as folders.
Some graphical shells (like the Program Manager of Windows <=3.11) represent launchable programs as folders.
Also, in some hierarchical lists, items marked with folder pictures contain other files.
(I am under the impression Apple Lisa used folders first - or did Xerox PARC invent them?)