http://sheldonbrown.com/fixed.html#flipflop
Fixed/Free Flip-flop hub means that only one side is threaded for a lockring since a freewheel doesn't need a lockring. The moving part of the freewheel is built into the sprocket, not the hub.
edit: d beat me to it
Last edited by prooftheory; 11-01-12 at 07:31 AM.