Natural harmonics occur every time a tone is struck. With musical instruments, depending on the note, an entire series of waves sound, each moving at different frequencies, from very high to very low. Just as different notes are used to make up a chord in music, several waves are used to created just a single tone. Many people who describe the color or timbre of an instrument, whether it is crisp like a bugle or dark like a cello, are basing their descriptions on the natural harmonics that occur when that instrument is played.
Tap harmonics are specific to fretted stringed instruments. Although they technically can be created by banjos and mandolins, they are used most prominently by electric guitar players. This harmonic is created through a technique in which the player, instead of strumming, uses his or her finger to tap the string against the fretboard. Since the metal frets come into contact with the nickel-wound strings, and the noise is so amplified, a burst of harmonic overtones occurs. Each fret, when tapped properly, will create its own specific harmonic, so many players use this as a method of composing pieces of music.
Like tap harmonics, pinch harmonics are common to stringed instruments. Although the tap harmonic is not limited to fret-based instruments, it is most common with electric guitars. This harmonic is created when a player plucks a string while slightly muting it with the edge of the thumb immediately after. The result is a cutting off of much of the low-frequency waves of the harmonics, leaving the tone to consist solely of high-piercing overtones. Many players who use distortion with their sound are able to create clear and distinct pinch harmonics and have incorporated this effect into their style of play.