Tremolo is a musical technique used on various instruments, which calls for rapid repetition of a single note or chord in order to create a sense of agitation. It is produced in very different ways on different instruments.
On string instruments such as the violin, the performer moves the tip of the bow rapidly back and forth over the string, while a single note is held on the fingerboard. While not often used in solo violin literature, it is a ubiquitous effect in symphony orchestras, especially in opera.
On a guitar, a tremolo is produced easily with a pick, which is run back-and forth across the string. It is a distinctive effect used often by surf guitarists such as Dick Dale (most memorably in "Miserlou"), as well as by Heavy Metal guitarists. On the classical guitar, it is a much more difficult effect which is produced by holding the hand perpendicular to the strings, and plucking the string with alternating fingers.
On the piano, tremolo on a single note is seldom used (though in harpsichord music its use was frequent), as it is difficult to execute and sounds more brilliant than agitated. One of the few piano works to incorporate this type of tremolo is the notoriously difficult third movement of Maurice Ravel's Gaspard de la Nuit. When the same effect as string instrument tremolo is desired on the piano, the pianist alternates as rapidly and smoothly as possible between two (or more) notes, often an octave apart. When done in the bass register of the piano, it creates a rumble like the timpani.