The threading for a fixed-gear has two sets of threads - threads for the cog, and threads for the lockring. The threading for a freewheel only has threading for the freewheel (which is typically the same exact threading for a cog).
A flip-flop hub is a hub that is threaded on both sides of the hub. What kind of threading it has depends on the manufacturer, though fixed/free is the most common. Fixed/fixed is available though and is generally preferable as you can thread a freewheel onto fixed threading (Technically you can thread a cog onto freewheel threading, but you won't be able to put a track lockring on to hold it in place).
Can't you thread on a fixed cog on the freewheelside and then thread on a bottom bracket lockring right behind it? I thought they were the same threads as a cog.
You can, but the threading on a proper fixed hub is a left hand thread to prevent unthreading. Using the BB lockring has the potential (however low is up to you) to become loose since they are both right hand thread.
Last edited by TurdFerguson2; 07-17-08 at 02:22 PM.