Generally speaking, it has baffles inside that trap the gases. See the associated link for diagrams. ADDING: Oh, and silencers are not really "silent" either. Most of them make a powerful gun sound...
(3) The term "firearm" means (A) any weapon (including a starter gun) which will or is designed to or may readily be converted to expel a projectile by the action of an explosive; (B) the frame or...