Vibrator is another word for oscillator. In other words, a crystal or other device used for generating a wave, as in an electrical (or electronic) circuit. This use of the word has declined due to the modern sexual connotations.
In the sexual context, there are a few major types of vibrators which have extreme popularity. Nearly all vibrators work on the same principle, which is that some electrical power source drives an electric motor, which is attached to an unbalanced counterweight. This causes an eccentricical motion. Vibrators can be used to stimulate any part of the body, and are not limited to penetrative female use.
The most obvious type of "vibrator" is the basic phallic vibrator, which comes in various realistic and non-realistic shapes, as well as unrealistic sizes. It is possible to get sleeves of various shapes, sizes, and textures which are intended to be fitted over vibrators, as well. Probably the most famous handheld vibrator in this category is the rabbit pearl, a vibrator of Japanese design named for its small rabbit-shaped protrudance designed for clitoral stimulation.
There are also a number of vibrators ostensibly designed for massage, in order to loosen tired muscles. Whichever way you use them, they certainly do that. The most popular of these is probably the Hitachi Magic Wand. Somewhat hammer-shaped (earning it a number of entertaining nicknames), this is an extremely powerful vibrator with multiple settings and attachments, which runs off of mains power - In the US, approximately 110VAC. This is the Cadillac of handheld vibrators.
A simpler, older, and more common type of handheld vibrator supposedly designed for massage has a profile much like a hand mixer. They have a pistol grip, more or less, and come with a variety of attachments. Supposedly they really are useful in massage. They are relatively inexpensive and long-lived. These have been around for "medical" purposes since the forties. They are interesting in that they are more useful than most for stimulating the head of the penis, and also are quite useful when applied to the clitoris.
There are of course various vibrators intended solely for the pleasuring of the male organ. Most of them are designed to mimic a vagina, and made out of some sort of pliable plastic or rubber.
Another famous vibrator is the Sybian. This is to a handheld model as a riding mower is to a push mower. The Sybian is supposedly designed to be the ultimate ride, and provide health benefits beyond those of simple stress relief. Whether this is really true or just hype remains unclear, but the girls in the videos look like they're having fun. At a (current) price of USD$1,315, one certainly hopes that they're not making it up. They continue to remain in business, so there is hope.
Another important type of vibrator is the small "personal massager". These are very small devices, usually cylindrical in nature, and sometimes equipped with interchangable rubber heads. One popular example is the Fukoku 2000, which turned up in a large number of places, including a myriad of television ads. A variation of these mini-vibes includes a finger strap; the bionic finger.
There are also a large variety of small pod-type vibrators shaped like eggs, thimbles, spheres, thumbs, and even dolphins, butterflies, and scorpions. These devices come with and without straps. The batteries and controls are stored in a separate box. Some of the more elaborate vibrators in this category have three or more motors all running at different (variable) speeds. Similarly, some vibrators are integrated into some type of cock ring, often with a protrusion designed to stimulate the partner's clitoris.
Some vibrators are designed for stimulation of the anus or even the prostate. Vibrating butt plugs, both purpose-built and as sleeves for a smaller, less descript vibrator are readily available, while vibrators with a small, oval-shaped vibrating node on the end of a long, slender stalk are also popular. Any sex toy inserted anally should have a base broader than its body which should be difficult or "impossible" to get past the anal sphincter. Otherwise there is a danger of "losing" the toy inside the rectum, with results ranging from humorous to medical emergency. This is especially true with vibrating toys, or any others containing batteries.
Anything with an imperfectly balanced motor, or a reciprocating motion can be used as a vibrator. Electric toothbrushes are popular. The ever-popular act of sitting on the washing machine when it is unevenly loaded turns a major appliance into a personal pleasure device. One woman I knew used an electric shaver — a waterproof model, of course — with the reciprocating cutting head removed. This left a small plastic nub with a beveled tip which moves back and forth rapidly and with great force; She covered this with a piece of silk bought from the fabric store. In addition to the moving nub acting on the clitoris, the overall vibration of the unit stimulates the surrounding area, especially the labia.
Vibrators may be made of a wide range of materials including hard plastic, plastic rubber, jelly rubber, silicone rubber, stainless steel, and glass. Of these hard plastic and plastic rubber (as opposed to natural rubber) are probably the most common, followed by jelly rubber and silicone. Of these the best materials are probably hard plastic, surgical stainless steel, and silicone. Jelly rubber is porous, and glass is easily damaged and it can be hard to detect when it has been chipped. Jelly rubber is popular because of the strong resemblance in feeling between it and skin but it is porous and can hold bacteria. Silicone is similar but non-porous and therefore can be cleaned very easily. Any vibrators can be protected from soaking up any undesirable fluids by covering them with a condom.