Found this these days:

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 This means that the file has been archived to tape and will take a very long time to access.
  [The Old New Thing]

Love to read Raymond Chen and learn about all the hidden, unknown and backgrounds on Windows. My favourites: What is in the "Windows 95 Special Edition" box? and Windows brings out the Rorschach test in everyone.