100/120 respectively.

Spacers work on to the hub if you have a standard sized frame and need a smaller wheel to fit. You add them to the axle and it 'lengthens' the axle. A fixed-gear hub is threaded the same way the older 5/6/7 freewheel (or any freewheel) hubs are. You screw a single cog (sprocket) on, and then a lockring on after.

The lockring has additional threading (before the freewheel threading, working from the outside of the hub in). Lockrings are reverse-threaded, which means to tighten you screw the opposite way of normal (it appears that you're loosening). Lockrings prevent the cog from unscrewing itself if you try to slow down, stop, or anything that puts back-pressure on the cog.