any fixed-gear bikes are equipped with "flip-flop" hubs, designed to accept sprockets on either side. These permit a choice of two different gears by removing the rear wheel and turning it around.
The most common use for a flip-flop hub is to have a fixed sprocket on one side, and a single-speed freewheel on the other side. Usually the freewheel will be 1 or 2 teeth larger than the fixed sprocket.
The idea is that, most of the time you would ride the fixed gear, but if you found your self far from home and getting tired, or were in unusually hilly terrain, you would turn the wheel around and use the freewheel. This helps two ways:
The lower gear will make it easier to climb the hills.
The freewheel will let you rest (coast) on the descents (which could be painful with the lower gear if it were fixed.)
Note that for each tooth difference, the axle position in the fork end will change by 1/8" (3 mm.)
Also, note that you should have two brakes if you will be using a freewheel.
You can also use two different sized fixed sprockets on a flip-flop hub. Generally I would recommend only one tooth difference in this case. I run 14 & 15 with a 42 front myself on a couple of my own bikes.
Most flip-flop hubs are only threaded for a lockring on one side, but the sprocket/freewheel thread is the same, so you can screw a fixed sprocket onto the freewheel side. I'd put the smaller sprocket on the side without the lock ring, because it's less likely to come unscrewed.
There are double-fixed flip-flop hubs, and, to me, this is the most desirable configuration. This arrangement is the most versatile, because you can set it up either with 1 or 2 fixed sprockets, or 1 or 2 freewheels.
Any standard track hub can also be used with a single-speed freewheel just by leaving the lockring off. The thread is the same. Sometimes people worry because the hub thread isn't as deep as a freewheel specific hub, but this is never a problem with a single-speed freewheel.