a fixed gear has a single speed, but singlespeed usually refers to a bike that has a freewheel - a mechanism that lets the cranks/pedals stop turning while the wheel still turns. like a ratchet. a fixed gear has no such mechanism, so if the wheel's turning, the chain and cranks and pedals are all turning. on a singlespeed, you can spin the cranks backwards. on a fixed gear, trying to do so actually pushes the rear wheel in reverse.