single speed - just like it sounds,one speed, with a freewheel, you can coast with this type of hub

fixie - a single speed hub that is "fixed" in place, no freewheel, so you cannot coast with this type of hub

flip/flop -hub with a single speed cog on one side and a fixed cog on the other - thus you can flip flop the wheel around depending on whether or not you wish to ride fixed or free.