Spermicides are sometimes used in addition to some methods of birth control. They form a chemical barrier at the cervix, preventing semen from entering the womb, and also immobilize spermcells. Spermicides come in several forms, like foam, cream, jelly and tablets.
It can be inserted into the vagina before intercourse when you use condoms, decreasing the risk of pregnancy.
When you use a diaphragm (or cervical cap) you place some spermicidal cream or jelly in the dome of the diaphragm, spreading it around the rim. You must do this before you insert the diaphragm, because the blocking effect of the diaphragm is not very reliable by itself.
Spermicides are in and of itself not enough to prevent pregnancy! They do not protect you against STD’s. With some girls they cause allergy and irritation (redness, itching, pain).