This method is about 98% effective.
A condom is a barrier device most commonly used during sexual intercourse to reduce the probability of pregnancy and spreading sexually transmitted diseases (STDs - such as gonorrhea, syphilis, and HIV). It is put on a man's erect penis and physically blocks ejaculated semen from entering the body of a sexual partner.
This is generally the most common form of contraception, and you should aim to have a few handy just in case some impromptu rolling in the sack occurs. The best place to store them is probably a bedside locker, that way they're easily accessible, yet out of the way as well. One of the worst places to keep them, and so many men in particular are guilty of this, is your wallet. A wallet takes a beating in its life, that is it bends constantly with movement or when a man sits down; not to mention the friction from opening and closing the wallet between transactions. It can cause a condom to deteriorate and you may not be able to tell if you look at the condom, though small tears and holes could be present, rendering it useless.
Also, make sure to check the expiry date on the condom as this can elude some people, resulting in STDs or unwanted pregnancy.