Its a threaded muzzle
All other forms of attachment are fine. If its sold as a flash hider, its verboten to install it on a post-ban rifle (write your US Represenative and Senators to urge Bush to NOT sign back into law the AW ban of '94).
Now, the distinguishing differences between brakes and hiders? Not much. My brake on my AR is very efective in hiding flash, especially from my baby loads. My AR is a Pre-ban, so I can experiment.
The other distinguishing characteristic is the OD of the flash hider. Some can use a NATO type grenade that captures the bullet as a trigger to arm the thing. Supposedly, one could use a flash hider of the AR15 (Pre-ban, of course) to launch such a grenade. Muzzle Brakes made to BATF circle-jerk regs are smaller diameter and supposedly, do not offer this possibility. Funny thing is I guess the Jack-Booted thugs (y'all can quote me on this one!) have never heard of masking tape.