What exactly do they do for the gun?
i heard it can effect bullet damage, true?

With a silencer you don't appear as a red dot on the enemies' radar while shooting.

When you attach the silencer there is no damage-loss shown so I think they don't effect the damage.

I'm pretty sure silencers reduce damage for ranged shots. Damage should be the same as not having a silencer for CQB. I may be mistaken though. ;P

The bigger the map, the more useful they become. You will die LESS using one.