A barrier physically prevents sperm from entering the womb.
The male condom is made from latex or polyurethane, and fits over the erect penis.
The female condom is made from polyurethane and is inserted inside the vagina before sex. Caps and diaphragms are each designed to fit over the female cervix.
All methods are most effective when used along with a spermicidal, because a spermicidal kills the sperm.