They fit inside your vagina and cover your cervix. Vaginal diaphragms are circular domes made of thin, soft rubber with a flexible rim. Cervical caps are smaller. New varieties of caps are becoming available made from silicone. To be effective, diaphragms and caps need to be used with a spermicide.
Spermicides are chemicals which destroy sperm.
How effective any contraceptive is depends on how old you are, how often you have sex and whether you follow the instructions. This means that between 4 and 8 women in 100 will get pregnant in a year. If they are not used according to instructions, more women will get pregnant. A diaphragm or cap stops sperm reaching an egg. It covers your cervix while the spermicide kills any sperm.