Traditional - also called "Rocket Vibrators" or "Smoothies" This is most basic style - straight up and down - manufactured from hard plastic and designed for insertion in the vagina. They usually have multi-speed controls and come in lots of designer colors and sizes. If you have never purchased a vibrator this is the place to start.
Vibrator Sleeves - made to slip over traditional vibrators, sleeves are manufactured out of rubber, jelly, silicone, or hi-tech cyberskin. They are designed to produce a number of versatile erotic sensations.
Massagers - not to be confused with the traditional vibrator, a massager is not designed for internal insertion and is primarily for tension relief, aid in body massages or clitoral stimulation. Most of the large electric vibrators you see fall into this category. Usually about a foot long, these vibes plug into a wall socket and have variable-speed controls. The unit consists of a long handle topped with a soft, vibrating head approximately the size and shape of a tennis ball. 
G-Spot - are specifically designed for female g-spot stimulation and will have a small crook at the top of the vibrator. Men, however, can use them for prostrate stimulation.
Jellies - are just traditional style vibrators manufactured out of jelly-like material. The advantage is that the jelly makes them softer to use.
Clitoral - come in two styles: wearable or hand-held and are designed specifically to stimulate the clitoris. While some can be inserted vaginally, these vibes should never be inserted anally. The wearable variety is sometimes called a "Butterfly". Designed to resemble a harness or pair of G-string panties, the butterfly style vibe is a small, intense vibrator held fast to a woman's clitoris for an intense, localized vibration.
Multi Function - these are multi-purpose vibrators and perform the function of a traditional vibrator and a clitoral vibrator. They have a shaft that thrusts inside the vagina while a clitoral nodule stimulates the clitoris. These may also be called Hi-Tech or Japanese Style. The Japanese Style have small animal shapes incorporated into the design plus vibrating pearls - the provide intense simultaneous clitoral, vaginal and G-Spot stimulation.
Remote Control - the control for this vibrator can be held by someone else and can even be at a distance. Provides lots of interesting sexual play for couples.
Waterproof - can be used under water (hot tub, shower, or bath) etc.
Contoured - the basic design resembles more of a woman's shape rather than the traditional phallic shape.
Egg or Bullet - designed to be placed inside the vaginal opening and come with battery packs for controlling the speed of the vibrations. 
Nipple Stimulators - provide the same kinds of vibrator stimulation for the nipple area.
Slims - same length as a "Classic Vibrator" but much thinner in diameter.
Minis and Pocket - small enough to slip in a purse or pocket but just as powerful as the normal size vibrator.
Acrylic - very popular due to their attractive appearance and non-porus surface.
CyberSkin - this revolutionary material looks and feels like real skin.
Anal - the physical design of the anal vibrator is smaller in length than traditonal vibrators and specifically for anal penetration.