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).