A singing bowl is a bowl that produces a ringing bell-like sound when the rim is stroked. You can produce exactly the same effect with a wine glass and a wetted finger. Singing bowls will produce a much stronger tone, and the strongest bowls will give note that will eat through your brain and drill holes through the fabric of space.
Singing bowls have a long but largely undocumented history in Buddhist tradition, going back at least a thousand years . They are used to help focus the mind in meditation, and for holding offerings on the altar. In China, Vietnam, and Japan the bowl can be used in counterpoint to chanting and playing on the wooden fish (AKA mokugyo). The traditional singing bowl is made from various bronze alloys, and may have very detailed engravings. The exact processes used to make the older singing bowls has been lost over time, but singing bowls of various qualities are still made and used worldwide. If you are an American trying to buy one you are likely to find bowls that look like a simple copper beggar's bowl, in keeping with the idea of simplicity in Buddhism.
The bowl is held lightly in the palm of the hand, and stroked with a mallet, usually a simple wooden pestle-like baton. The mallet may also be rubber tipped or wrapped in leather. One usually strikes the bowl lightly once, and then runs the mallet slowly around the inner edge of the bowl's rim. The bowl will shortly start to produce an intense but calming ringing sound. It may take some practice to get it right. Some bowls produce the best sounds simply by being struck, acting as standing bells. Either way, a good Buddhist singing bowl produces interesting harmonic overtones, giving it an eerie singing sound.
There is another modern incarnation of the singing bowl; crystal bowls, made from silicon glass or crushed quartz crystal, are an intense alternative to the traditional Buddhist bowls. While these bowls are also called singing bowls, they are a very modern invention, and make a different type of sound. They are carefully formed to produce a single clear note, with no harmonic overtones. This note is very, very intense and much less calming than the traditional metal bowls. The notes produced can sometimes be heard for miles, and will cause other bowls in the area that are tuned to the same note to reverberate with them. These bowls are 'played' in the same way as the traditional bowls, using a rubber-tipped mallet.
Crystal bowls are often used for sound therapy, in which a bowl is set over a persons body to allow the sound energy to enter directly into a person's body. Bowls tuned to different notes and placed over different parts of the body are said to stimulate different chakras. The language used to talk about the uses of crystal bowls often contain high levels of nuttiness, but whatever your views on the metaphysics involved, the bowls themselves are undeniably cool.