It's not suitable for a proper fixed gear wheel. A proper fixed gear wheel has threads as you have them on your wheel (the larger diameter side), but the threads are shorter. And then on the same side there are smaller threads that go in the other direction. Those are for a lock ring to secure the cog. You can still use your wheel as a fixed gear wheel, but you'll have to use a setup called "suicide" setup. For that you'll screw on the cog and than screw on a BB lockring on the same threads. Obviously this is not as safe as the proper setup.

Track/fixed gear hub (notice the second, smaller threads)

http://sheldonbrown.com/fixed-conver...l#freewheelhub

You probably have the second (BMX) type flip-flop hub