Diaphragms and caps are forms of contraception that prevent sperm from meeting and fertilizing an egg. They fit inside the vagina and cover the cervix (entrance to the womb). They can be made of rubber (latex) or plastic (polyurethane) and come in different shapes and sizes.
Vaginal diaphragms are circular domes with flexible rims. Caps are smaller than diaphragms. To be effective they need to be used with spermicide, which is a special cream that kills sperm.
Their effectiveness depends on how well they're used. When used according to the instructions, the effectiveness is usually greater than 90%.