A tuning fork is a metal instrument which is designed to vibrate at a particular frequency. It usually has a handle with a rounded ball on one end (so you can touch it to another object without scratching the other object). Attached to the handle are two "tines" -- usually square in cross-section, and often quite long. The steel is springy, so if you hit the fork, the tines will move together and apart and together and apart again. Here’s a picture of a set of fourteen of them. The little ones have higher natural frequencies than the bigger ones.
They are used mainly by musicians to help tune their instruments. Only a small fraction of people can hear a note and tell you exactly what the pitch is. A much larger fraction of people can tell you which of two notes has a higher pitch than the other. If a musician falls into the latter category, then an absolute standard is needed every time he tunes his instrument, and a tuning fork is a handy thing to carry around -- all you need to do is tap it a little, and touch it to a sounding board, or just hold it up to your ear, and compare it with the note produced by the instrument.
These days, tuning forks may be a little old-fashioned. Electronic tone synthesizers are much more flexible (you can get only one pitch out of a tuning fork, but cheap electronic box will make lots of different ones for you for about the same price). But some musicians prefer tradition.
(published on 10/22/2007)