Spermicide comes in various forms: suppositories, foam, cream, jelly, and film (thin sheets). All work by immobilizing and killing sperm. It's often used in conjunction with barrier methods of birth control such as the condom, cervical cap and diaphragm. If you choose, you can use it solo and it's 71-82% effective at preventing pregnancy.
You need to place spermicide in the vagina close to the cervix before sex so it has time to melt and become active. Possible side effects are vaginal irritation and allergic reaction to nonoxynol 9, a common ingredient in most spermicides.