A female condom is similar to a male condom. The only difference is the female version is inserted on the inside, while the male version is on the outside of the body. The condom keeps the sperm from entering the woman's body.
Female condoms are just as effective as male condoms if used correctly all of the time. On average, it is 75%-82% effective. There are two rings, one on each end. The closed end goes first and the open end of the condom covers the vulva. This can result in pregnancy for the same reasons a male condom results in pregnancy. If it rips, not wearing one every time you have sex, and spilling of semen when removing the condom.
The female condom protects you from sexually transmitted infections. If your partner does not want to wear a condom but you do not want to take a chance of getting pregnant, using the female condom would be the best idea. Meanwhile, in order for them to be effective, you have to use them every time you have sex or it will eventually stop working as protection.