There are two kinds of firearms under U.S. (federal) law, Title I firearms and Title II Firearms.  Title I firearms include long guns (rifles and shotguns), handguns, silencer, and firearm frames or receivers.  Most NFA (national firearms act) weapons are also Title I firearms.  Title II Firearms are NFA Weapons.  Title II of the 1968 Gun Control Act is the National Firearms Act (codified at 26 U.S.C. sec. 5801 et seq.), thus the nomenclature "NFA".  Title I is generally called the Gun Control Act, (18 U.S.C. sec. 921 et seq.).  NFA weapons are also sometimes called Class 3 Weapons, because a Class 3 SOT (special occupancy tax) is needed to broker NFA weapons.

These weapons may also be further regulated by states or localities, and while these weapons can be legally owned under federal law, some states and localities further regulate ownership or prohibit it.  The NFA Branch of ATF administers the taxation of the guns, and the registration of them in the National Firearms Registration and Transfer Record.

NFA weapons are: machine guns, sound suppressors (silencers), short barreled shotguns, short barreled rifles, destructive devices and AOW's (any other weapons). Exactly what these weapons are is defined in the law, as well as in court cases interpreting the law. For further answers to questions about NFA weapons, make a visit to your local FFL/SOT Holder.