a compression system is something that is KEY if you are riding threadless. Without it, there is nothing to keep your fork, headset, and bars, from just, sliding off the scooter. On threaded, there is a locknut. On threadless, there is a choice of just 3 at the moment: ICS (Internal Compression system, there is a starnut in the bars, and a big long bolt that goes through your fork, and into the bars. This keeps it all together), SCS (Standard Compression system, a "clamp" that holds your forks, and your bars. There is a starnut in your fork, and a bolt keeps it all together), and HIC (Hidden Internal Compression, simple, but effective. a shim that goes on top of the fork. there is a starnut in your fork aswell, and a bolt that goes through the headset top cap, and through the starnut, keeping it all together).
SCS: Most expensive, strongest (No slit in bars).
HIC: Cheap, Very Effective (but you NEED OVERSIZED bars, and OVERSIZED clamp). Comes stock on MGP nitro's.
ICS: Cheapest (most of the time). Not that great, but does the job. It's great if you are good with it.