A membranophone is any musical instrument which produces sound primarily by way of a vibrating stretched membrane. It is one of the four main divisions of instruments in the original Hornbostel-Sachs scheme of musical instrument classification, the other three being idiophone, chordophone and aerophone (a fifth class, electrophone, was added in a later revision).
Most membranophones are drums. Hornbostel-Sachs divides drums into three main types: struck drums, where the skin is hit with a stick, the hand, or something else; string drums, where a knotted string attached to the skin is pulled, passing its vibrations onto the skin; and friction drums, where some sort of rubbing motion causes the skin to vibrate (a common type has a stick passing through a hole in the skin which is pulled back and forth).
In addition to drums, there is another kind of membranophone, called the singing membranophone, of which the best known type is the kazoo. These instruments modify a sound produced by something else, commonly the human voice, by having a skin vibrate in sympathy with it.