Instruments are categorized by how they produce sound.  Instruments are classified using 5 different categories depending on the manner in which the instrument creates the sound: Idiophones, Membranophones, Chordophones, Aerophones, & Electrophones.

Hornbostel-Sachs classifcation system, idiophones are further divided into struck idiophones and plucked idiophones.

Membranophones are instruments that make sound from the vibrations of stretched skins or membranes.
Drums, tambourines, and some gongs are common examples of membranophones.

An aerophone (/ˈɛəroʊfoʊn/) is a musical instrument that produces sound primarily by causing a body of air to vibrate, without the use of strings or membranes (which are respectively chordophones and membranophones), and without the vibration of the instrument itself adding considerably to the sound (or idiophones).

chordophone, any of a class of musical instruments in which a stretched, vibrating string produces the initial sound. The five basic types are bows, harps, lutes, lyres, and zithers. The name chordophone replaces the term stringed instrument when a precise, acoustically based designation is required.

Membranophones

Sound is produced by the vibration of a tightly stretched membrane.

Struck – drums (many varieties)

Plucked – plucked drums (a string is attached to the membrane and causes the vibration)

Friction – friction drums (rubbed rather than struck or instruments in which a cord is attached to the membrane and rubbed)

Singing – kazoos

Unclassified

Bowed instrument

Bowed instrument

Chordophones

Bowed Instrument

Sound is produced by the vibration of a string or strings that are stretched between fixed points.

Simple/Zither – musical bows, zithers

Composite – lutes, harps, tube fiddle, violins, viola, cello, bass, guitars

Unclassified

Aerophones

Sound is produced by vibrating air.

Free – early organs, accordion, harmonica

Non-free – flutes, recorder, oboes, clarinet, saxophone, trumpet, trombone, euphonium, tuba

Unclassified

Electrophones

Sound is produced by electric action or amplification.

Instrument Classification

Hornbostel-Sachs instrument classification system was created by Erich Moritz von Hornbostel and Curt Sachs. Among ethnomusicologists, it is the most widely used system for classifying musical instruments.

Instruments are classified using 5 different categories depending on the manner in which the instrument creates the sound: Idiophones, Membranophones, Chordophones, Aerophones, & Electrophones.