a fixed hub is where your hub is only able to accept the peddaling direction you peddle in. a free hub just enables you to stop peddaling/ peddle backwards without your bike braking or wanting to go in reverse. if your bike did not have a free hub at all, front or back, when u stopped peddaling or peddled backwards your wheel would either brake or go backwards. old school bmx;s did not have free hubs and to brake u simply stopped peddaling, this locked up the back wheel. front or rear freehub depends on your choice, but from what ive seen you can get better quality hubs with a rear freewheel, so i wud go for that. that dosnt mean its a better choice its just that rear freehubs seem better kit and have been proven on BMX for years