Condoms do help prevent pregnancy and sexually transmitted infections (STIs). Each year, 85 out of 100 couples who have sex but don't use any form of birth control get pregnant. That number drops to only 15 out of 100 when condoms are used. And no other method of birth control is as successful at protecting people against STIs as condoms.
Condoms are most effective at protecting against STIs like HIV/AIDS, gonorrhea, and chlamydia. Condoms can also protect against genital warts (HPV) and herpes, but are less effective against these because warts and herpes can show up in areas that are not covered by a condom.
The only way to be 100% sure that you won't become pregnant or get an STI is to not have sex at all (called abstinence). But even couples who practice abstinence can benefit from learning about condoms. One study shows that a quarter of the couples who try to abstain from sex get pregnant in their first year together.
Condoms are easy to get and use. Anyone can walk into a store and buy them. It's a good idea to have a condom readily available because they're a good birth control choice for people who haven't planned ahead.
Reprinted from http://kidshealth.org/teen/sexual_health