http://blogs.msdn.com/oldnewthing/archive/2003/08/27/54710.aspx
Windows XP ships with a number of icon overlays.
A small arrow. Everybody knows this one: It's the shortcut overlay.
A hand, palm up. This is the "sharing" overlay. A folder with this overlay
is the root of a file share.
A downward-pointing blue arrow. This is the "to be written to CD" overlay.
A pair of blue swirling arrows. This sometimes baffles people. This means
that the item is available offline. (You have to enable offline folders to
get this.)
A black clock. This really baffles people. This means that the file has been
archived to tape and will take a very long time to access.