A diaphragm is a latex ring that has a cup attached to it, and that cup is a reservoir for spermicide. What you do with this cup is that you fill it with spermicide, and you have to carefully place that into your vagina so that the cup covers the cervix, and that is how you prevent pregnancy.
The diaphragm is a very particular type of birth control. You have to place the diaphragm correctly in order to prevent pregnancy. You have to use spermicide on the inside cup of the diaphragm in order to get the best contraceptive effectiveness. This requires a little bit of skill on the part of the user, because it's not as easy as it looks to place this cup into your vagina with the spermicide, over your cervix, before you have an act of sex.
The diaphragm is an effective form of birth control. However, it is not available like condoms. You need to be fitted for the diaphragm by a nurse practitioner or a physician, and you need to get a prescription after you're fitted to purchase this diaphragm at the pharmacy.
So if you are interested in using the diaphragm, you need to contact your nurse practitioner or your physician so that you can discuss your birth control options and that you can be properly fitted for this form of birth control.