Silencers -- actually sound suppressors, since a truly silenced weapon has to be built that way from scratch -- are legal under Federal law ASSUMING you have a license to possess such a thing. The same is true of fully automatic weapons (machine guns, submachine guns, full auto assault rifles, Uzis, Tommy guns, machine pistols, etc.). Suppressors reduce the sound of a shot, they do not eliminate it.

Wikipedia has a nice discussion of "silencers".

Please note that you CANNOT, because of the mechanics of the thing, use a suppressor on a revolver. So the next time you see the gangster screwing a "silencer" onto his snub-nosed .38 revolver....

I cannot see any value in using them in hunting. They may have some use in poaching.

As a side note, I should mention that it is quite simple to make a single use or few uses suppressor. If you really want to know look it up -- like making explosive devices at home I ain't gonna tell you how.