This is such an important question, as there are a few types of materials: latex, polyurethane, polyisoprene and lambskin. So, be sure to check the packaging before you get it on to know for certain what you're using. If the packaging or wrapper isn't specifically labeled, it's likely a novelty condom only intended as a gag gift--so reach for something else.
Latex condoms are effective at preventing pregnancy and reducing STD risk if they're used consistently and correctly every time. They're available in a wide selection of sizes and textures. But know that they can only be used with water- or silicone-based lube, as products that contain oil (like petroleum jelly, baby oil and body lotion) will cause them to deteriorate.
Some people, however, are allergic to latex, which may cause them to experience a rash, itching, burning or swelling. Often, people discover a latex allergy while at the dentist, when latex gloves are being used in their mouth. Since this tissue, like the vagina, is made of mucus membrane, the latex can cause a similar reaction during sex.
Polyurethane condoms primarily serve as a good alternative for those that are allergic to latex, rather than as a go-to type. Polyurethane condoms are also effective at preventing pregnancy and reducing STD risk if they're used consistently and correctly every time. Typically, they're less elastic with a looser fit, transfer body heat better, and can be a bit noisy with each thrust. They can be used with water-, silicone- or oil-based lube.
Polyisoprene condoms are newer to the market. They too are effective at preventing pregnancy and reducing STD risk if they're used consistently and correctly every time. The material is a synthetic version of latex, so they're a good alternative for those that have latex and polyurethane allergies. The fit is similar to a latex condom, they are soft in texture, and they too can only be used with water- or silicone-based lube.
Lastly, lambskin condoms are only effective in reducing the risk of pregnancy. While they feel more "natural" since they are made from the intestinal lining of a lamb, they are porous--meaning they have microscopic holes--which while small enough to prevent sperm from passing through, are large enough for bacteria and viruses to be transmitted. So, they should only be used if you're in a monogamous relationship and not concerned about STDs.
Remember--with the exception of lambskin--condoms only protect what they cover and act as a barrier between.
Click here for some tips on how to properly put on a condom.
Be sure to also check out this video to learn other essential tips to use a condom correctly.