A barrier birth control method is anything that physically blocks the sperm from travelling through the uterus to meet up with the egg. Condoms (external/male and internal/female), cervical caps, and the sponge are all considered to be barrier methods. All barrier methods must be used before any sexual contact begins to be most effective.
Condoms are the most effective type of barrier method and they are also they easiest to find. They also help to reduce the risk of STIs.