What is a dental dam and why should I use one?
Dental dams are small, thin, square pieces of latex that are used for oral sex. They get their name from their use in dental procedures. Dental dams help to reduce the transmission of STIs during oral sex by acting as a barrier to secretions that contain bacteria and viruses. They come in a variety flavors - so you can find a dam that satisfies your tastes.

How effective are they in preventing STIs?
Because dental dams act as a barrier to bodily fluids, they help reduce STI transmission. Many STIs, such as herpes, genital warts and HIV, can be transmitted through oral sex. Like condoms, dental dams must be used correctly and consistently in order to be effective.

How do I use them?
Although it may seem a little awkward to use them at first, dental dams are extremely easy to use. Before using the dam you may want to rinse off any powder that's on the dam and check the dam to make sure there are no holes or perforations. The partner performing oral sex will hold the dam against the genitals of the receiving partner. You can opt to apply a lubricant on the genitals before using the dam. The lubricant can help increase the sensation for the recipient. Just make sure the lubricant is a water-based lube because oil-based lubes and lotions can degrade the latex and decrease the dam's effectiveness.

When you use the dental dam, be sure to ONLY use one side. Don't flip the dam over for another round because you will expose yourself to the very fluids you're trying to avoid! And do not re-use a dam on another body part because you can transfer germs from one body area to another. Do not re-use a dam for another act of oral sex later on either. Dams are for one-time use only.

Won't using a dental dam diminish the whole experience of oral sex?
Many people believe that using a dam will nullify the enjoyment of oral sex. That's not how it has to be! Granted, the feeling of latex will be different than a tongue, but that's not necessarily a bad thing. Using a dental dam with lube can offer your partner a new type of stimulation. Although oral sex is considered less risky than vaginal or anal sex, there is still a risk of transmitting STIs. To be as safe as possible, use a dental dam for every act of oral sex.

(information provided by brown.edu)