A quick trip to google would do you some good, but since I am here, I will share some. A fixed gear bike utilizes a cog and lockring to keep the rear wheel in sync with the pedals at all times. The cog has standard threading, and the lockring has a smaller reverse threading. This makes it so that when you go to pedal backwards the cog doesn't just fly off. On a fixed gear bike there is no need for a back brake because you can apply pressure to slow yourself down. A front brake is always recommended for emergencies.

On a single speed bike this cog and lockring setup is replaced with a freewheel. This is a mechanism that can only act on pressure applied in a forward motion. This means that you can coast, or even pedal backwards while the rear wheel is unaffected. These bikes should always have a front and rear brake

Many bike have what is known as a flip-flop hub so that each of these can be used just by flipping the wheel around. This is nice if you are tired on a long ride and are in a hilly area. Usually a lower gearing is applied to the freewheel side (usually 1 or 2 teeth bigger cog) so that hills can be climbed more easily.

Look some more up on google and pay a visit to sheldon brown's website. Hope this helped