Male condoms are an effective form of birth control. However, about 1 out of 50 couples who use condoms correctly will get pregnant in a year. Chances of pregnancy increase if you don't always wear a condom during intercourse, or you use condoms incorrectly.
Condoms are effective at preventing the transmission of most STIs, although there's still some risk. When used correctly, a condom creates a barrier that limits your exposure — and your partner's exposure — to semen or other body fluids that can carry STIs.